Music Practice Changes Your Brain

How does a life with or without music practice change your brain? Photo by T. Gaertner

When I have kids who are struggling with their piano pieces, I try to encourage them by acknowledging that playing the piano is hard. In fact, by learning how to play music, they are actually changing the structure of their brain.

But how much am I exaggerating here? How much does musical training really change your brain?

There’s some pretty good evidence to support the fact that musical training causes people to have larger auditory cortex, a larger hand area in the motor part of their cortex, and better connections between these two areas and between the two sides of the brain. This research has generally been done two different ways: a) by comparing the brains of musicians to those of non-musicians and b) by giving music lessons to children and seeing how their brain structure changes.

Both of these types of research have the same problem: we can’t tell what differences are caused by practice and what are caused by genetics. Perhaps people who have brains that grow larger auditory cortex (for example) are the ones who have a natural talent for music, and so they continue with lessons and become musicians. In other words, maybe it was the brain structure that caused the person to be a musician, not the other way around. Maybe the kids who stick with music lessons are the ones whose brains are genetically predisposed to being “musical”.

What would be really useful is some magical way of taking a single person and seeing what their brain would look like with and without a lifetime of musical practice.

A recently-published study has managed to do the next best thing. The researchers, Örjan de Manzano and Fredrik Ullén, at the Karolinska Institute in Sweden, studied 9 identical twin pairs in which one twin studied piano and the other did not. In each pair, both twins started music study at the same time but one dropped out of lessons quite early while the other continued with piano lessons. On average, the piano-studying twin had played the piano for over 4000 hours more than the twin who quit piano, and the playing twin was still an active amateur pianist.

The researchers looked to see what differences in brain structure were found between the practicing and non-practicing twins, assuming that if the piano-playing twin hadn’t practiced, their brain would look like their twin’s (which is a reasonable assumption, based on other twin studies). Basically, the non-practicing twin acted as a control for the practicing twin.

What they found confirmed some of what had been found in previous studies: musical training leads to a thicker cerebral cortex in auditory and motor areas on the left side of the brain and a greater volume of grey matter in parts of the cerebellum, which plays a role in motor control. Musical training also increases the organization of white matter in the auditory and motor parts of the cortex on both sides of the brain and in the corpus callosum, which connects the two hemispheres.

These changes in brain structure are found in the parts of the brain most used in piano practice. When we practice, we’re connecting auditory information with motor control, linking up the movements that we need to make in order to produce audible musical sounds. In the brains of musicians, the auditory cortex, the motor cortex, and the fibre bundles that connect these regions are bigger and better organized. Also, the corpus callosum, which plays a role in bimanual co-ordination, has more organized structure. These differences in brain structure between piano-playing and non-playing twins are clearly not due to genetic differences, so they must be due to differences in life experience. In other words, hours and hours of music practice have altered the structure of the playing twins’ brains.

This result does not mean that genetics has no role in musical ability. Other research from the same lab at the Karolinska Institute has suggested that amount of practice is less important to musical expertise than genetic factors. Our genetics not only influence the fluidity of our motor control and our ability to discriminate pitches and keep a beat, but also shape how much we’re willing to practice, our ability to focus, and our emotional and motivational responses to music. But, as this twin study clearly reveals, our day-to-day experiences with music also play an important role. The question is not whether musical ability is shaped by nature or nurture; as with almost everything, both play a role. The interesting question is how nature and nurture interact to mold our abilities.

So yes, music practice really is changing your brain. Of course, music study is not necessarily unique in this regard. Anything you spend a lot of time doing is going to change your brain. For example, expert basketball players surely have different brains too. But not all skills lead to long-term, large changes in brain structure. In fact, there is a theory that learning a new skill causes brain growth at first, but then as the skill is solidified, brain structure is renormalized and returns to its original size. The fact that this doesn’t seem to happen with musical training suggests that playing an instrument depends on a difficult set of skills requiring specialization of a number of parts of the brain. Mastering these skills does not come without effort.

As for my struggling students: Knowing that this is true doesn’t make it less work, but perhaps it can be reassuring, and motivate them to keep growing the auditory-motor parts of their brains, synapse by synapse.

References:

de Manzano, Ö., and Ullén, F. (2017). Same Genes, Different Brains: Neuroanatomical Differences Between Monozygotic Twins Discordant for MusicalTraining. Cerebral Cortex 1–8.

  

Wenger, E., Brozzoli, C., Lindenberger, U., and Lövdén, M. (2017).Expansion and Renormalization of Human Brain Structure During Skill Acquisition. Trends in Cognitive Sciences 21, 930–939.

Mosing, M.A., Madison, G., Pedersen, N.L., Kuja-Halkola, R., and Ullen,F. (2014). Practice Does Not Make Perfect: No Causal Effect of Music Practice on Music Ability. Psychological Science 25, 1795–1803.

Crossing the Midline

More and more these days, people are interested in the neuroscience underlying our behaviours and our ability to learn. This is fantastic, but the downside to this enthusiasm for neuroscience is that there is a lot of pseudo-neuroscience making the rounds. A few weeks back, when someone on a piano pedagogy Facebook group mentioned exercises for “crossing the midline”, a warning sign immediately flashed in my mind, and I decided to investigate a little.

There is a fairly common neuro idea that exercises that involve body parts crossing the midline (for example, touching your right elbow to your left knee) are good for encouraging neuroplasticity, especially interhemispheric communication (the two sides of the brain talking to each other) and bilateral sensory integration (putting together sensory information from the two sides of the body). Midline-crossing exercises are touted to improve how the two sides of the brain talk to each other and have all sorts of other benefits. See, for example, this article entitled “Why crossing the midline activities helped this child listen to his teacher”.

 My gut feeling was that these claims are questionable, and I wanted to know whether there is any research to support them.

A quick search of the scientific literature found exactly zero studies investigating this effect. There is no evidence to directly support the idea that crossing-the-midline exercises improve interhemispheric communication. A Google search of the same topic turned up a 2013 “Ask a neuroscientist” blog post, which confirmed that there is no research to directly support the effectiveness of crossing-the-midline exercises.  In fact, that blog post suggested that the best way to increase connections between the two hemispheres of the brain was to learn a musical instrument.

It’s tempting to just conclude that this idea is pseudoscience and leave it at that. But given the prevalence of this idea, I’d like to dig just a little deeper and talk about where this idea comes from. There is actually some logic to it. To start with, let me explain how these types of exercises work both sides of the brain. Imagine you’re using your right hand to reach over and touch a target in front of the left side of your body. The right hand is controlled by the left motor cortex. Your awareness of space on the left side of your body happens in the right side of your brain, in the posterior parietal lobe.

If we move our right hand into the left side of our personal space, then in order to coordinate where our hand is in space, the left motor cortex must communicate with the right parietal lobe, using fibres that travel through the corpus callosum, the big fibre bundle connecting the two sides of the brain. 

Presumably, if both sides of the brain are active and talking to each other, this increases the strength of their connection, but I should reiterate that it’s not clear that simply performing exercises that cross the midline will lead to increased connectivity.

When I delved into the literature about crossing the midline, I found that researchers study the development of midline reaching as part of the development of handedness.  If you put an object in front of a baby, but put it a little to one side of the midline, she will almost always reach with the closest arm.  As children age, they become more likely to reach with the dominant arm, which means that if the object is placed on the non-dominant side, they must reach across the midline to pick it up.  The development of this behaviour seems to parallel the development of the corpus callosum, the big bundle of nerve fibers which communicates between the two hemispheres.  This doesn’t necessarily mean that the cross-midline reaching causes interhemispheric communication; it’s more likely that the increase in communication between the sides allows the hemispheres to specialize and this leads to hand dominance.

Children with developmental delays often don’t automatically reach across the midline.  They are more likely to reach with whichever hand is closest to the object. This is correlated with delayed dominance and decreased laterality of the brain.  So occupational therapists do test for a child’s tendency to cross the midline, and if the child doesn’t reach across the midline in a normal fashion, the therapist will recommend midline-crossing activities to try to help develop a more dominant hand. This doesn’t seem to be an evidence-based therapy, seeing as there aren’t any studies to support it.  However, therapists may see improvement based on these exercises in individual cases and this justifies their use.

Even if midline-crossing exercises do help develop hand dominance and bimanual interaction in children with developmental delays, that doesn’t mean there is any benefit to these exercises in normally developing children.  In children who have developed strong hand dominance, midline-crossing exercises probably aren’t doing much, in my opinion. 

In short, there is no evidence that these exercises are useful for people with normal development, and even for people who are not neurotypical, the main relevance seems to be in the development of handedness, not some miraculous creation of connections in the brain.

References:

Provine, R.R., and Westerman, J.A. (1979). Crossing the midline: limits of early eye-hand behavior. Child Dev 50, 437–441.

Schofield, W.N. (1976). Do children find movements which cross the body midline difficult? Quarterly Journal of Experimental Psychology 28, 571–582.

Schofield, W.N. (1976). Hand Movements Which Cross the Body Midline: Findings Relating Age Differences to Handedness. Perceptual and Motor Skills 42, 643–646.

Surburg, P.R., and Eason, B. (1999). Midline-crossing inhibition: an indicator of developmental delay. Laterality 4, 333–343.

Surburg, P.R., Johnston, J., and Eason, B.L. (1994). Effects of midline crossing on response processing of adults with mental retardation. Journal of Developmental and Physical Disabilities 6, 327–338.

 

Aging and Motor Memory

In my spare time, I am slowly learning Bach’s English Suite in A minor, movement by movement. I take a break from writing or studying and I sit down at the piano, pick a small section, and work through it, trying to get it under my fingers. I always make progress throughout a practice session, and leave the piano feeling like I’ve accomplished some learning. But the next day, when I come back to the piano, and try the same section, I’m usually disappointed. The improvements from the previous day don’t seem to last in the same way they did when I was a younger musician.

Unfortunately, this is a natural effect of aging.

Memory and learning differ between young adults and older adults in a number of ways. Older adults tend to have more difficulty with declarative memory – conscious memory for facts and events – than younger adults. This is often attributed to age-related loss of neurons in the hippocampus, a structure in the brain that is the site of declarative memory formation. So if I tell you that Ulaan Baatar is the capital of Mongolia, you would store that in your declarative memory. If you’re twenty, you are more likely to remember this fact tomorrow than if you are seventy. From a psychological point of view, deficits in declarative memory seem to be related to decreased attentional resources in older people. In other words, older people can’t pay attention to as many different things at once as a younger person can, so they can’t spend as much time committing any one fact to memory. That means the capital of Mongolia won’t be stored in their memory as stably as it would in a young person.

Motor learning uses a different type of memory known as procedural memory, and has its own difficulties for older adults. Older people are able to improve at a new skill over a practice session, but they show a different pattern from young people when it comes to retaining that skill. When young research volunteers learn a new motor skill, such as a finger-tapping pattern, they improve significantly over a practice period, with a decrease in the number of errors and a gradual increase in speed. When the volunteers come back the next day for a second session, their performance often has improved overnight, without any further practice. This is due to sleep-dependent consolidation, in which our motor skills both improve and become resistant to interference from other memories, while we’re sleeping. People can literally improve their motor skills, such as playing a musical instrument, just by sleeping.

As we age, things start to change. Older adults just don’t show the same between-sessions improvement in motor skills, and in fact their performance on the second day of training on a task starts out much lower than where they left off the day before, just like when I practice Bach. When researchers compared the brainwaves of sleeping young adults with sleeping older adults, it became clear that older adults spend less time in slow-wave sleep and show a decrease in sleep spindles, a particular type of brainwave that is believed to be important in motor memory consolidation.

Research from the University of Montreal suggests that the hippocampus, even though its main role is in declarative memory, is important for sleep-dependent consolidation of motor memory. This implies that decreased hippocampal function in aging leads to problems with motor memory consolidation. The key point here is that there are important interactions between the declarative and procedural memory systems. Which means that the declines in declarative memory which happen naturally with age also affect procedural memory.

Understanding this gives us a hint at a solution to the problem of motor learning in older adults. One of the ways in which we learn motor skills is by using declarative memory to help us along the way. For example, when we’re learning a new piece of music, we consciously read the music and try to be aware of things we need to remember:  cross finger 4 over here; F# in left hand here, and so on. Using declarative memory to bolster motor learning is a poor strategy when declarative memory isn’t working so well. In order to counteract the effects of poor declarative memory on motor learning, we should choose practicing strategies that rely more on implicit, procedural memory, strategies based on repetition of the movements we want to learn rather than our cognitive appraisal of the notes and movements required to make them.

The obvious candidate for this type of learning is a technique called errorless learning.  This technique suggests that if you can simplify a task somehow so that it can be practiced without making errors (or at least as few as possible), then you engage procedural memory systems, leading to more automatic performance. For example, a 2012 study by Chauvel and colleagues tested older and younger adults in two techniques to learn golf putting skills. One group used “infrequent error” learning, where the people practiced putting into a hole from a short distance away. The other group practiced putting from a larger distance, while led to more frequent errors because the task was harder. This second group had to develop declarative strategies about how to improve. Then both groups were tested on putting from an intermediate distance, with and without distractions. The researchers wanted to see if the older adults fared better with one type of learning than the other. And the results were clear: when using the “infrequent error” approach, older and younger adults performed equally well on the test. Using the “frequent error” approach older adults performed worse than younger adults.

In music practice, errorless practice can be achieved by practicing at a slow enough tempo to avoid mistakes in pitch and rhythm. I’ve adopted this approach recently, playing everything extremely slowly and accurately and I found that it improved my retention of the pieces both within and between practice sessions. 

Of course, the idea of practicing slowly is not new or mind-shattering. I often tell my students that if they’re practicing their pieces so quickly that they’re making mistakes, then they’re actually practicing their mistakes. But since reading about errorless learning, I’ve been encouraging them more and more to practice error-prone sections using “slow-motion” practice, and this has been very helpful for them. It’s revealing to see the reasoning behind why this works: because errorless practicing is training our procedural, automatic memory.

While my sense of pride wants me to point out that I don't actually fall into the category of "older adult", I'm not as young as I used to be, and clearly that makes a difference in how my memory processes function. As in the story of the tortoise and the hare, slow and steady wins the race, especially if you're no spring chicken. 

References

Albouy, G., King, B.R., Maquet, P., and Doyon, J. (2013). Hippocampus and striatum: Dynamics and interaction during acquisition and sleep-related motor sequence memory consolidation. Hippocampus.

Chauvel, G., Maquestiaux, F., Didierjean, A., Joubert, S., Dieudonné, B., and Verny, M. (2011). Use of nondeclarative and automatic memory processes in motor learning: how to mitigate the effects of aging. Gériatrie et Psychologie Neuropsychiatrie du Viellissement 455–463.

Chauvel, G., Maquestiaux, F., Hartley, A.A., Joubert, S., Didierjean, A., and Masters, R.S.W. (2012). Age effects shrink when motor learning is predominantly supported by nondeclarative, automatic memory processes: Evidence from golf putting. The Quarterly Journal of Experimental Psychology 65, 25–38.

Craik, F.I.M., and Rose, N.S. (2012). Memory encoding and aging: A neurocognitive perspective. Neuroscience & Biobehavioral Reviews 36, 1729–1739.

Fogel, S.M., Albouy, G., Vien, C., Popovicci, R., King, B.R., Hoge, R., Jbabdi, S., Benali, H., Karni, A., Maquet, P., et al. (2014). fMRI and sleep correlates of the age-related impairment in motor memory consolidation. Hum Brain Mapp 35, 3625–3645.

King, B.R., Fogel, S.M., Albouy, G., and Doyon, J. (2013). Neural correlates of the age-related changes in motor sequence learning and motor adaptation in older adults. Front Hum Neurosci 7, 142.

 

Teaching composing to children: Q&A with Frances Balodis

My students of all ages, even the three-year-olds, have spent the last few months lovingly creating their own musical compositions. The impetus for this is provided by the Music for Young Children (MYC®) International Composition Festival, which is celebrating its 30^th anniversary this year. Thousands of MYC students from around the world have sent their compositions to be played and reviewed by a panel of teachers and composers who are charged with the difficult task of deciding which compositions will make it to the final round and be given first, second, or third place ranking, or an honourable mention.

I recently spoke over the phone with Frances Balodis, the founder of MYC® and chair of the MYC Composition Festival.

TG: Why do you think it’s important to teach composition to young children?

FB: It helps them understand what they’re playing. It helps them memorize. When they come to memorize something you can say, “There’s the motive, and now it’s repeated, but is it repeated exactly the same? Now listen to the sequence.” So when they go to play by memory, and they falter just a little bit, you can support them so nicely by referring to the compositional techniques.

Also, when they are composing, you can talk about the need to have dynamics and tempo markings. You can ask them, “Would you like the whole piece to be allegro, or is there going to be a ritardando?”

I have noticed that many children really improve in their playing after studying composition, and they improve in their understanding. After we’ve done composition, then the children will look at a song that they’re going to play, and say, “Oh I see the motive.” I think that by teaching composition, it really opens up their eyes to what they’re playing.

TG: What do you think is most challenging about composing for the students?

FB: Keeping the whole map in their head, because sometimes they will start out with a really good idea and then they kind of go off on some side trips and they have a little trouble getting back home. It’s important for them to understand how to take a trip and explore lots of really interesting things, and come back home.

TG: What makes a good composition?

FB: I like to see an interesting motive. And you can have an interesting motive even if you only know C, D, and E. And coming to a good conclusion, a conclusion that makes sense. I think variety is also important. Sometimes you get a composition where the left hand is all broken triads, too repetitive. I was looking at Facebook this morning and saw a darn good composition that someone had posted. They had a nice waltz pattern, and then they changed the left hand pattern so they had a nice little broken chord. That contributes to being a good composition. A good composition also has nice phrasing, and sensible cadences. 

TG: For the composition festival, the children write out their compositions in their own handwriting. How important is that?

FB: Honestly, some of the compositions that my children wrote, I did not have them spend hours and many tears recopying them. I think that’s a mistake, and it makes me sad when people send their compositions in and they look spick and span. And I think, “Hmmm… I hope the child didn’t cry when they had to recopy it.”

What I used to say to my students is that writing a composition is like writing a letter. When I send you a letter, in my handwriting, as long as you can read it, the communication has been successful. It is not successful if I send you a letter that you can’t understand. You have to be able to look at the letter that I’ve sent you and understand what I’m trying to tell you, and it’s the same thing with a composition. When we make our composition and we send it off, we’re sending a musical letter.

I always tell the reviewers of the compositions, if the treble clef is backwards, if the stems are on the wrong side of the note, it’s okay. Sometimes the winning composition looks like a chicken walked across the page.

TG: What is the best way to introduce composing to young children?

FB: I like the concept of teaching composition through art. I got the idea of doing that and then people kept saying, “Oh gosh, I wish this was written down.” And then Frederick Harris published my Young Composers Notebooks for quite a number of years.

With the youngest children I just use coloured circle stickers. You can move the circles up the page, you can move the circles down the page, you can make the circles go backwards, which sometimes is enough — just to teach the children the compositional techniques of repetition, sequence and retrograde.

Bach is such a wonderful example of these techniques: there it is, there’s the motive, there’s the repetition, there’s the sequence. Bach was the master of sequence. So sometimes when children or parents say, “That’s too easy,” I say, “Really? Take a look at the masters here. It’s not too easy.”

This interview has been edited and condensed.

A Tale of Two Sight-Readers

Aria is 10 years old, and has been studying piano with me for many years. She’s small for her age, with a shy, soft voice and a wry sense of humour. At her lesson, I test her note-naming and find it, to be perfectly frank, abysmal. We continue on with the lesson, and start work on a new piece, Beethoven’s Ecossaise in G Major. I ask her to sight-read the right hand part, and she peers at the music, figuring out what the first note is. Then she plays through the first line of music fairly well, doing an excellent job considering her terrible performance at note-naming. However, in the second bar, there is an interval of a seventh. She misreads the second note of the interval, ending up playing the end of the phrase a step too low.

Despite not knowing her notes very well, Aria is a half-decent sight-reader because she mostly reads by interval, simply going up or down one note (a step) or two (a skip) as the music indicates. Larger intervals are harder to read, so she sometimes makes mistakes with these, and reading over a line break is much more difficult because it’s hard to see the vertical relationship between the notes when they are on different lines.

Leonard, 9 years old, arrives for his lesson with a cheerful smile. He has been learning The Silent Moon by Nancy Telfer, but when he plays it for me, he quickly runs into trouble, hitting a wrong note in the second phrase. He can hear it too, and restarts the phrase, this time landing on a different wrong note in the same place. I notice that Leonard is not looking at the music; he is watching his fingers. Despite the fact that the music book is open on front of him, he is playing from memory, searching for the notes by ear. I’m concerned about his ability to read the music, so I test his note-naming using stack of flashcards. Leonard names the notes quickly and easily, so I open up his sight-reading book to a simple exercise. Before he plays it, I ask him to look at the music and tell me how many times the melody moves by a skip instead of step. He puzzles through the music and incorrectly tells me there are three skips in the melody. In fact, there is only one.

These two students use entirely different strategies for reading music. Leonard does well at reading by note, while Aria’s strength is reading by interval. Strong sight-readers combine the two strategies. What you might not realize is that the two different strategies use entirely different parts of the brain.

When we read music, the visual information travels from the eyes to the visual cortex at the very back of the brain. The visual cortex receives that information and then passes it on to other regions of the visual cortex that process that information, sorting out contours of the things we see, how they’re oriented in space, if they’re moving, what colour they are, and how bright. The brain needs to do two main things with all this information: It has to answer the question “what am I seeing?” and it has to answer the question “what should I do with what I’m seeing?”

To answer these two questions, visual information is processed through two very different pathways in the brain, known as the ventral stream (vision for perception) and the dorsal stream (vision for action). 

Dorsal stream (red arrow) and ventral stream (blue arrow) of visual processing. Image courtesy of "BodyParts3D, © The Database Center for Life Science licensed under CC Attribution-Share Alike 2.1 Japan."

The ventral stream, which involves the temporal lobe of the brain, has long been known as the “what” stream, since it is the pathway we use to recognize and name the things we are seeing. It is this pathway that we use for note-naming. Imagine we see a note on the first space of the treble staff. In the ventral stream, what we see match up the image of what we see with pictures we have stored in memory, and this is how we know to name that note as a F. This is how we recognize what we see, leading to a conscious perception of what we are looking at, and a conscious understanding of what we see. In music reading, we use the ventral stream for note-naming, recognizing musical symbols and understanding their meaning, for conscious pattern recognition, and for naming chords.

It’s this ventral stream that is a weakness in Aria’s sight-reading. She has a hard time naming individual notes, so finding the correct note to play at the beginning of a line or after a large interval is difficult. Leonard, on the other hand, has a strong ventral stream, but on its own it is not enough to make him a good sight-reader. He also needs to have a strong dorsal stream.

The dorsal stream, which involves the parietal lobe, directly relates what we see to the actions that are required. The parietal lobe plays an important role in spatial perception, and so this pathway processes the spatial aspects of what we’re looking at, and matches it up with our knowledge of what to do with that object. This activates the correct movement. In music reading, we use the dorsal stream to know what movements to make to play straight-forward patterns, to know how far to reach for each interval, to make the correct hand shapes for chords. The dorsal stream automatically converts well-known visual cues into movements.

This is why Aria, who is terrible at note-naming, can sight-read music pretty well. Her dorsal stream does a good job of reading intervals and telling her what finger movements she should make. Leonard, on the other hand, tries to rely on his ventral stream for reading music. He can name the notes well, but he doesn’t easily translate the written notes into how he should move his fingers.

How do I help these two students? It’s not enough to just hand them a sight-reading book and tell them to go practice. Aria needs to practice note-naming specifically:  matching up notes on the staff with their names. In addition, she should practice playing individual notes on the piano. Flashcards are a good tool here. Leonard has different needs:  he should practice reading intervallically. An excellent resource for this is The Sight Reading Drill Book by Barbara Siemens, which systematically introduces intervals and chord patterns, and encourages the student to read by interval and by hand shape rather than by note-naming, strengthening the dorsal stream of visual processing. Siemens describes this approach by saying, “It’s a mind-finger thing. I think sometimes you have to try to bypass that naming thing and just do it intuitively. Which means you have to drill it enough.” As an experienced piano teacher, Siemens saw a need in her own students for intervallic sight-reading practice. “Because you don’t have time to think of notes as you’re going.  The name thing is just attaching a tag to something that should go intuitively.”

Every student has different strengths and weaknesses. This is true even within a single skill such as sight-reading. As a teacher, it’s important that I remember that and use different approaches to bolster students’ abilities and help them achieve their musical goals.

References

Goodale, M.A. (2011). Transforming vision into action. Vision Res. 51, 1567–1587.

Goodale, M.A. (2013). Separate visual systems for perception and action: a framework for understanding cortical visual impairment. Developmental Medicine & Child Neurology 55, 9–12.

Goodale, M.A., and Milner, A.D. (1992). Separate visual pathways for perception and action. Trends in Neurosciences 15, 20–25.

Ungerleider, L.G., and Mishkin, M. (1982). Two Cortical Visual Systems. In Analysis of Visual Behaviour, (Cambridge, Mass.: MIT Press), pp. 549–586.