Part Of: Neuroanatomy sequence
Content Summary: 1000 words, 10 min read

Blindsight

Have you heard of blindsight before?

This man has no conscious experience of vision. And yet, he can avoid obstacles while walking!  Blindsight is not a trick, and hundreds of cases are documented. How can a person see, but not know that he sees?

To answer, we turn to the brain.

Two Visual Streams

Recall our concept of flat map. Cortex is like a sheet: its wrinkles can be stretched flat.

Green areas are primary areas: landing sites of different sensorimotor modalities. The visual primary area is V1, somatosensory touch is S1, muscles is M1.

Visual information is pumped from the retina to V1. From there, the Dual Stream Hypothesis suggests that it follows two distinct pathways:

The traditional graphic illustrates how visual information passes up into the dorsal stream, and down into the ventral stream.

Things become more clear with a flat map (right). Consider the phenomenon of hand-eye coordination involved in, e.g., playing table tennis. Moving the paddle to return a serve require close collaboration between visual information (about the ball) and motor information (from the hand). Here, it seems clear that hand-eye coordination is provided by the dorsal stream, which comprises the shortest distance between V1 and M1.

Blindsight patients have damage to V1. They are not conscious of visual experiences, yet their ability to maneuver obstacles suggests that their hand-eye coordination (Dorsal stream) is still intact. How can this be?

Well, the optic nerve also passes information directly to the dorsal stream. While the Ventral Stream of V1-damaged blindsight patients receives no information, the Dorsal Stream remains functional.

Inspired by these blindsight patients, Milner & Goodale gave these streams nicknames.

• The Dorsal How Stream performs hand-eye coordination
• The Ventral What Stream produces conscious perception.

Our subjective experience of unitary vision is wrong!  If you step back, though, it makes perfect sense for natural selection to carve out two flavors of vision, each with different purposes. Action-based vision needs to happen quickly; perceptual-based vision is more analytical and doesn’t have the same speed requirements. 

Tension In The Dorsal Stream

The Dual Stream Hypothesis is grounded in strong neuroscientific and behavioral evidence. It also has enjoyed consensus support from neuroscientists for nearly two decades. However, there are also some notes of tension.

Conceptual tension to a researcher is blood to a shark.

Did you know there are actually two versions of the Dual Stream Hypotheses? Goodale & Milner’s 1992 version (explored above) is the most well-known, but in 1982 Mishkin et al also proposed a Dual Stream model, based on monkey lesion studies. Both accounts agree about the Ventral What Stream. But Mishkin’s model gives a spatial processing role to the Dorsal Stream (“Where”). Why should these research traditions view the Dorsal Stream so differently?  Note well such cases of functional tension.

Let me advance two observations that are unique to myself.

First, consider the width (angular spread) of the two streams. In the above flat map, notice how much wider the Dorsal Stream is than the Ventral Stream (nearly three times wider!).  How can the Dorsal Stream be so wide, yet retain functional coherence? This is structural tension. While such hints are less damning, they destabilize our already queasy relationship with the Dorsal Stream.

Second, it is curious that roughly ⅓ of the cortical  area around V1 are simply not recruited in the Dual Stream Hypothesis. Surely such regions (e.g., Lingual and Cuneus cortex) have some role in the processing of visual information. Could these medial regions comprise a fourth stream?

Call this the Medial Stream Conjecture. The medial stream seems to project directly the hippocampus. I suspect that in five years, we shall speak of the Medial Experiential Stream, which supports autobiographical memory. 

The Lateral Stream: Healing The Divide

A parable of three disciplines:

• The cognitive psychologist speaks the language of function. How does the mind create behavior?
• The neurophysiologist speaks the language of structure. How do anatomical minutiae participate in neural circuitry?
• The cognitive neuroscientist serves as translator. She pursues structure-function maps; the connective tissue between biology and information processing.

Sometimes these maps go awry, and must be repaired. The Dorsal Stream → { What or Where } map is such a case.

Several researchers have proposed that the Dorsal Stream be split into two separate streams (e.g., Rizzolatti & Matelli, 2003). Call this the Three Stream Hypothesis. The “true” Dorsal Stream is more narrow, and retains its How functionality. However, the remaining cortical surface is now called the Lateral Stream, which performs spatial processing (Where). 

Milner’s Dorsal How Stream was much too wide. Conversely, Mishkin’s “Dorsal” Where Stream is actually located more laterally.

The Lateral and Dorsal streams both connect posterior parietal to premotor cortex. Two decades ago, these areas were largely referred to by name  (“the premotor cortex does X”). Modern treatments, however, parcellate these regions at a much finer granularity. This also helps explain how Milner and Mishkin conflated the streams.

Integrating Audition

Originally, the dual stream hypothesis was viewed as only relevant to vision. However, lately there has been an influx of interest in auditory streams. The seminal paper is Hickok & Poeppel (2007), The cortical organization of speech processing. It proposes two streams:

1. The antero-ventral stream (green, left) performs auditory classification, and assists in speech comprehension.
2. The postero-dorsal stream, (red, right) in contrast, is not bilaterally symmetric.
   1. The postero-dorsal stream in the right hemisphere localizes auditory stimuli both spatially and temporally.
   2. The postero-dorsal stream in the left hemisphere performs speech production.

The auditory stream model integrates cleanly with the Three Stream Hypothesis. The auditory antero-ventral stream shares real estate with the Ventral stream. The auditory postero-dorsal stream overlaps the Lateral stream. 

Why is “deafhearing” (an auditory version of blindsight) impossible?  Because ear-hand coordination is not a thing. Auditory information simply does not participate in the unconscious Dorsal stream.

Summing Up

The Three Stream Hypothesis describes how auditory and visual information are carried as far as the prefrontal cortex. We may carve each stream into three discrete phases:

Finally, we can also visualize these same relationships more abstractly, as follows:

Until next time.