10.6 Visual Processing in the Brain

After studying this section, you should be able to:
- 10.6.1 Describe evidence for separable systems analyzing what we see and where it is, and relate this evidence to the case of D.F. from the opening of the chapter.
- 10.6.2 Name the cortical region that detects motion in the visual field, and describe the results of damage to this area.
- 10.6.3 Describe mirror neurons and how they might relate to behaviors like empathy and autism.

The primate visual system consists of numerous distinct regions.
Some regions specialize in processing different attributes of visual experience such as:
- Shape
- Location
- Color
- Motion
- Orientation
Despite the existence of over 30 visual fields, this number exceeds the number of basic visual attributes.
A hypothesis suggests that the abundance of distinct visual regions may be due to investigators focusing on the visual system due to their visual orientation as primates.

Early studies with hamsters led to the proposal of two visual systems:
- Object Identification (involving the visual cortex).
- Location of Objects (involving the superior colliculus) (Kravitz et al., 2011).
Mortimer Mishkin and Leslie Ungerleider (1982) suggested the existence of two main cortical processing streams in primates stemming from the primary visual cortex:
- Ventral Processing Stream (What Stream): Responsible for visually identifying objects.
- Dorsal Processing Stream (Where Stream): Responsible for perceiving the location of objects and guiding movement towards them (associated with optic ataxia).
- Recently, Ungerleider proposed a potential third pathway dorsal to the what stream specialized for social perception (the Who Pathway) analyzing face-related information (Pitcher and Ungerleider, 2021).

Patient D.F. provides insight into the functions of the ventral and dorsal streams:
- After carbon monoxide poisoning, D.F. lost the ability to perceive faces and objects but retained the ability to reach and grasp under visual control.
- Investigators hypothesized that D.F.'s ventral visual stream was damaged while her dorsal stream remained intact.
- Damage was confirmed via MRI, showing concentration in the ventrolateral occipital cortex indicating atrophy characterized by shrunken gyri and enlarged sulci.

Figure 10.29A: Reconstruction showing damaged brain region in D.F.
Figure 10.29B: Comparison of fMRI activation in healthy controls versus D.F. while viewing objects.
In D.F., while reaching, fMRI activity in the parietal lobe was normal, indicating the dorsal stream was functional.

D.F. can accurately grasp objects but cannot verbally identify them, highlighting the separation of the ventral and dorsal streams and how the brain usually integrates multiple sources of visual information effortlessly.

The perception of visual motion is analyzed by specialized systems, particularly in cortical area V5 (MT).
Some retinal ganglion cells respond preferentially to certain directions of motion (e.g., left vs. right).
Area V5 is integral to motion perception:
- PET studies show activation of V5 in response to moving stimuli in humans (Zeki, 2015).
- Experimental lesions to V5 in monkeys impair their performance in reporting motion direction (Cohen and Newsome, 2008).
Stimulating V5 neurons can alter perceived motion direction, demonstrating direct influence on perceptual experience.

A classic case involved a woman post-stroke who lost the ability to perceive motion due to damage to area V5 (Zihl et al., 1991).
- She experienced difficulties in comprehending dynamic scenes, leading to challenges in crossing streets and following conversations due to inability to interpret moving visual stimuli.

The anterior part of the dorsal stream merges with the motor cortex, harboring neurons with both visual and motor functions.
These mirror neurons were discovered while studying the motor cortex of monkeys:
- They fire in response not only to a monkey's actions but also when observing another perform similar actions.
- Proposed Role: Mediates understanding of observed actions, activating similar neuronal representations in the observer's brain (Rizzolatti and Sinigaglia, 2016).

Evidence from human studies supports the mirror neuron system, suggesting involvement in:
- Empathy
- Learning by imitation
- Evolution of language
- Implications of damage to the mirror neuron system may help explain conditions such as autism, characterize by difficulties in understanding social behaviors (Reynaud et al., 2019).

Reflect on the decision of which visual stream one would prefer to lose: the dorsal or ventral stream, considering their respective functions in visual perception and interaction with the environment.