Lecture 7 - Occipital and Temporal Lobes: Visual Perception and Memory

Question 1

How is visual information processing hierarchical?

Answer 1

The processing of visual information by the brain is hierarchical, with the complexity of the visual representation increasing from retina to visual association cortices and beyond (what the neurons in these areas experience becomes more and more complex but more accurately depicts our actual experience)

Question 2

How does the processing of visual information reflect functional differentiation?

Answer 2

At the different stages of information processing, there is functional differentiation, with different neuron types or different brain regions processing different properties of visual stimuli

Question 3

What are simple features of visual information processing?

Answer 3

• Light intensity and wavelength (e.g. cones)
• 2D position in visual field (different parts of the visual field activate different parts of the retina)

Question 4

How are simple features combined and elaborated?

Answer 4

Via parallel channels (in medial temporal lobe)

Question 5

What are complex visual representations for perception and memory?

Answer 5

• Integrated information concerning form, surface (colour, texture), spatial relationships, and movement
• Integration with other sensory modalities (multimodal representations) e.g. faces and names

Question 6

What is visual processing like in the extrastriate cortex?

Answer 6

• Neurons in extrastriate occipital cortex (area outside striate cortex) signal ‘global’ properties of visual scenes and objects, rather than ‘component’ properties
• Holistic signalling of form and motion in V3 and V5

Question 7

What is global colour vs. component wavelength?

Answer 7

• Perceived colour of an object depends not only on the wavelength reflected by object, but also on wavelength reflected by the surroundings (colour constancy, e.g. perceived colour of object does not change when viewed during sunset/in different lighting conditions)
• Some neurons in V4 are ‘colour’-sensitive (i.e. respond to wavelengths in the centre of their receptive field, depending on the wavelengths reflected from the background), whereas neurons in primary visual pathway and V2 are only ‘wavelength’-sensitive

Question 8

What is global/pattern motion vs. component motion?

Answer 8

• Primary visual cortex – neurons signal component orientation
• Complex neurons triggered by moving stimuli
• Only tell direction object is moving in due to V5 (perception of motion)

Question 9

What are visual information processing streams?

Answer 9

• Following V1 (and perhaps earlier) visual information processing is mediated by two streams that are anatomically and functionally differentiated

Question 10

What is the dorsal stream?

Answer 10

Visuo-spatial (‘where’)/visuo-motor (‘how’) processing – parietal cortex – integration of visual and motor information

Question 11

What is the ventral stream?

Answer 11

Object analysis (‘what’) – inferior temporal lobe

Question 12

What does the evidence for visual streams by Mishkin et al. (1982) show?

Answer 12

• Inferior temporal lobe lesions (‘ventral stream’) in macaques (monkeys) impair object-discrimination/recognition (‘what’), but not object location (‘where’)
• Posterior parietal lesions (‘dorsal stream’) impair object location (‘where’), but not discrimination (‘what’)
• Mishkin et al. (1982)

Question 13

How does Milner and Goodale’s ideas about the visual streams differ from Mishkin’s?

Answer 13

Milner and Goodale proposed that the ventral stream processes visual information for object perception (‘what’), whereas the dorsal stream processes visual information for visuo-spatially guided action (‘how’) – theory about dorsal stream different to Mishkin et al.

Question 14

What is the key evidence for Milner and Goodale’s theory?

Answer 14

Patients with occipito-temporal brain damage show severe forms of visual agnosia (i.e. deficits in aspects of visual perception without blindness), but intact visually guided actions, whereas patients with posterior-parietal lobe lesions show optic ataxia (i.e. deficits in visually guided reaching) with otherwise relatively intact visual function

Question 15

What does the evidence from patient DF show?

Answer 15

• For example, patient DF with extensive bilateral ventral-stream lesions has profound visual agnosia, but shows intact visually guided reaching (posting slip experiment Milner, 1998)
• DF can act on visual stimulus (e.g., visuomotor posting) because dorsal pathway is intact, but is unable to make perceptual judgements (e.g., perceptual orientation matching)

Question 16

What is the role of the inferior temporal cortex in visual perception and memory?

Answer 16

• The inferior temporal cortex receives inputs from extrastriate cortex and forms the final stage in the visual processing hierarchy of the ventral stream

Question 17

How do neurons in the inferior temporal cortex respond to shapes and objects?

Answer 17

• Neurons in the inferior temporal cortex can respond very selectively to specific shapes and objects.
• These responses can show:
• invariance to changes in size, orientation, and other properties – i.e., the neuron ‘recognizes’ object regardless of the viewpoint
• sustained activity in absence of visual object, reflecting short-term object memory

Question 18

What did the study by Miyashita and Chang (1988) show about visual perception and memory in the inferior temporal cortex?

Answer 18

• Neuron in TE responds to fractal shape in stimulus regardless of size, orientation and colour
• Other fractal shapes fail to trigger strong responses
• Sustained response during retention delay on matching-to-sample task (retention delay: 16s)

Question 19

What are face cells?

Answer 19

Some neurons in the inferior temporal lobe show highly selective responses to individual faces (very specific responses to very specific stimuli)

Question 20

What did the study by Perret et al. (1988) show about face cells?

Answer 20

Monkey’s neurons respond more strongly to PS than DP’s face

Question 21

What have the highly selective properties of face cells been compared to?

Answer 21

The highly selective properties have been compared to those of ‘gnostic units’ or ‘grandmother neurons’, i.e. hypothetical neurons at the end of a processing hierarchy that ‘recognize’ individual entities, such as your grandmother (although face cells typically respond to several faces; also compare Quian Quiroga, 2016, Neuropsychologia, concerning an evaluation of the ‘grandmother’ neuron concept)

Question 22

What has been identified in the human inferior temporal lobe?

Answer 22

Areas showing selective responses to faces have also been identified in the human inferior temporal lobe using functional imaging (e.g., in an area called the Fusiform Face Area) (Kanwischer N, Yovel G, 2006, Phil. Trans. R. Soc. B 361:2109)

Question 23

What is the role of the medial temporal lobe (MTL)?

Answer 23

• MTL is at end of visual-processing hierarchy, combining inputs from ventral and dorsal stream, and receives additional inputs from other sensory modalities
• It is thus in position to elaborate visual representations further and to generate multi-modal representations

Question 24

What are some examples of complex representations mediated by MTL structures?

Answer 24

• Complex spatial representations, requiring the encoding of relations between many visual stimuli
• Multimodal representations of experiences (‘episodic’ memory) and facts (‘semantic’ memory) (together referred to as ‘declarative’ memory)

Question 25

How does Patient H.M. provide evidence that the MTL is involved in processing multimodal information?

Answer 25

- Surgical removal of hippocampus and of parts of the surrounding cortices to stop epileptic seizures - Following surgery, HM showed severe and pervasive deficit in remembering new and recent experiences, facts, and places, whereas other cognitive functions, including procedural learning, were largely intact - These findings triggered enormous research activity on function of hippocampus and surrounding cortices

Question 26

How have the findings related to H.M and the hippocampus been confirmed in animal studies?

Answer 26

- Confirmed in animal studies e.g. selective place learning deficits after hippocampal lesions in rate (RGM Morris et al., 1982) – spatial learning is a multimodal task - Hippocampal place cells (in human hippocampus during virtual navigation)

Question 27

Encoding of multimodal percepts by hippocampal neurons

Answer 27

- Hippocampal neurons code for complex multimodal percepts - Neuron responds to images, name and voice of Oprah Winfrey (but not respond to other famous people in the same way) - Combines multimodal information about Oprah Winfrey

Question 28

Conclusions

Answer 28

- Perception and memory based on visual (and other sensory) information can be understood as a hierarchically organized sequence of processing steps mediated by interconnected brain networks - At the earliest stages neurons respond to very basic features (e.g. presence/absence of light, location in visual field) - At progressively higher stages, neurons respond to combinations of basic features and get activated by more and more complex stimuli - Visual information processing is also characterized by functional differentiation, i.e. different properties of visual stimuli are processed in parallel by different neuron types/brain regions (e.g., colour and motion; information concerning stimulus identity vs. information relevant to what to do with a stimulus)