Lectures 20-27

Success of vision depends on

Localisation of light reflected off distant objects in reference to the individual and environment
Form of object identification based on size, shape, colour, past experience
Movement detection
Compensation for changes in lighting conditions

Wavelength of light that is visible to humans

400-700nm

Rods are ... more sensitive to light than cones

1000x

Why is visual perception most accurate at central vision (fovea)?

No bipolar or ganglion cells - light strikes photoreceptors directly
Higher concentration of cones
Less blood vessels clouding vision

Ophthalmoscopic view of the retina

View through the pupil

Optic disk

In nasal hemiretina
Where optic nerve exits
Where blood vessels enter

Which retinal ganglion cell axons cross at the optic chiasm

Nasal retina

Retinotectal pathway (subcortical vision)

Retina - superior colliculus

Retinogeniculate pathway (cortical vision)

Retina - LGN - V1 (medial occipital lobe)

What are the axons leaving LGN called in the retinogeniculate pathway?

Optic radiation

Phosphene threshold

Minimum TMS intensity that evokes phosphenes

A higher phosphene threshold =

Reduced V1 excitability

Oliveri and Calvo (2003)
Ecstasy and V1 excitability

Ecstasy users had significantly lower phosphene threshold than controls - more responsive V1
Phosphene threshold among hallucinators was significantly lower

Maunesll and Van Essen (1983)
Single cell recording in area MT (homologous with V5)

Neurons fired vigorously for movement of bar in specific direction and at a particular speed.
Neurons in V5 are selective for direction and speed of motion

Zeki et al. (1991)
PET activation during visual stimulation

Method: PET was used to measure blood flow to see which parts of the brain were active for colour and motion vision.
Colour processing = ventral
Motion processing = dorsal

Where does the dorsal stream go

Parietal lobe

Where does the ventral stream go

Temporal lobe

Goldberg and Wurtz (1972)
Single cell recordings from visual neurons in the superior colliculus

Monkey trained to fixate on central position - stimulus presented in periphery - single cell recording used to map field
Conclusions: superficial layers contain retinotopic maps of contralateral hemifield

What layer of the SC do retinal ganglion cells input information

Superficial layers

Sprague Effect

Visual orienting responses can be restored in the cortically blind hemifield by removing the contralesional SC or by cutting the fibres that connect the two SC

Which SC needs to be disinhibited in the sprague effect?

Ipsilesional

Schneider (1969)
Visual cortex vs SC damage in rodents

Disruption of cortical vision impaired object discrimination
Disruption of subcortical vision impaired ability to orient to stimulus
Conclusion: Subcortical = basic orienting, cortical = more complex object discrimination

Perimetry testing

Seeing if people are responsive to certain areas of testing. Involves presenting a small spot of light at random locations across the visual field while patient fixates on centre. Patients report whether they detect the light

Weiskrantz (1986)
Residual vision without primary cortex

Task: When tone sounds, move eyes to location of light
Results: When spot of light appeared up to 20� into hemianopic field, responses correlated with position of light.
Conclusion: Blindsight, stimulus in the cortically blind hemifield activated retinote

Rafal (1990) - Tested whether distractors in cortically blind hemifield would affect speed of response to stimulus in intact hemifield

Reaction times were slower in the presence of the distractor - competing activation via retinotectal pathway
Conclusion: visual processing is occurring despite lack of awareness

Wurtz and Goldberg (1972)
Activity of deeper layers of the SC

Cells in deeper layers fired before and during eye movements.

Movement fields of deep layers of the SC

Part of the contralateral visual field to which eyes move in response to activity in the cell
Code for movements into the same area as retinotopic map in superficial layers

Size of saccades in caudal and rostral deep layers of SC

Caudal = large saccades
Rostral = fixation cells

Pierrot-Deseilligny et al. (1991) - Case study
Effects of unilateral SC damage on reflexive saccades

Latencies of reflexive eye movements were asymmetrical, contralesional saccades were delayed
Conclusion: SC plays a role in generating saccades toward stimuli in the contralateral hemifield

Fixation offset paradigm

Task: fixate on centre, when stimulus appears in periphery look at is asap
Overlap trial: fixation stimulus remains present
Offset trials: fixation stimulus disappears when target appears

Fixation offset effect

RT overlap trials - RT offset trials

Why do overlap trials have a slower RT in the fixation offset paradigm

Mutual inhibition of fixation and saccade cells

Large fixation offset effect indicates...

Strong fixation reflex

Ro et al. (1997)
Effects of TMS over cortex on saccadic eye movements
Methods

Endogenous task: move eyes left and right in response to central arrow
Exogenous task: move eyes to peripheral asterisk
TMS pulse delivered on each go signal - coil placed over superior prefrontal cortex and superior parietal lobule

Ro et al. (1997)
Effects of TMS over cortex on saccadic eye movements
Results and Conclusions

Endogenous results: TMS over SPFC delayed contralateral saccades, TMS over SPL had no effect
Exogenous results: Neither TMS of SPFC or SPL affected latencies
Conclusions: Delay in contralateral endogenous saccades associated with TMS over the SPFC was a c

Subcortical control of eye movements

Retina - SC - saccade generators (in the brainstem)

Cortical control of eye movements

Retina - Frontal eye field (superior prefrontal cortex) - saccade generators (brainstem)

Henik et al. (1994)
Effect of lesions involving FEF on voluntary saccades

Endogenous task
Patients with FEF lesions: voluntary saccades directed towards contralesional hemifield were delayed
Conclusion: FEF is normally involved in generating voluntary saccades

Hood and Atkinson (1993)
Fixation offset effect in infants

Compared to older infants 1.5 month olds showed significantly slower responses on fixation overlap trials
Conclusion: maturation of cortico-subcortical pathways may underpin the shift from predominantly exogenously controlled orienting to predominantly en

Anti-saccade task

Fixate on centre, when stimulus appears in periphery look in opposite direction.
Measure errors and reaction time

Antisaccadic experiments in children: age related performance
Fischer et al. (1997)
Munzo et al. (1988)

Rapid decrease in errors (60%-22%) from age 9-15
Children aged 5-8 years make the most errors

Machado et al. (2004)
Effect of lesion involving FEF on antisaccades

Patients with unilateral FEF damage made abnormally frequent reflexive eye movements towards contralesional visual signals (but not ipsilesional.
Conclusion: may reflect impaired modulation of the activity in the ipselesional SC

Overt shifts in attention

Involves movement of eyes

ERP experiment in neurobiologically healthy adults: effects of covert attention on activity in the occipital lobe

A stronger neural signal occurred in response to the stimulus when attention was directed (covertly) at location of stimulus

Exogenous attention task

Maintain fixation on centre, when stimulus appears in periphery press button asap. Some trials are cued by a flashing box

Endogenous attention task

Maintain fixation on centre, when stimulus appears in periphery press button asap
Before each stimulus an arrow appears at centre, on most trials it indicated where stimulus would appear

Inhibition of return

When attention is directed toward a location and there is a long delay before the target appears, the latency to respond to that target is increased when the target appears at the attended location compared to when the target appeared at a different locat

Reason for inhibition of return

Once your attention has been directed to a certain location and processing has occurred, your brain then inhibits attention to that spot in order to facilitate search in other areas

Flanker task

Measures how easily distracted a person is by external, distracting information

Flanker effect

RTs on incongruent trials (target and distractor are different) - RTs on congruent trials

Development of the brain

Cell division
Cell migration
Cell differentiation

Formation of ocular dominance columns

At birth: inputs from LGN representing the two eyes are intermingled within V1
After some sensory experience: inputs from two eyes segregate into ocular dominance columns

von Metlchner et al. (2002)
Rewiring the brain in newborns: evidence for functional plasticity

Rewired cell axons from retina-SC to retina-MGN-A1
Result: neurons in A1 acted like visual neurons of V1

Gothe et al. (2002)
Effect of stimulating visual cortex in adults with impaired vision

Measure self-reported phosphenes elicited by TMS
Effect of activating visual cortex via TMS is altered in people with severe visual impairment, those born blind could not experience phosphenes

Lambert, Sampaio, Mauss and Schneiber (2004)
Mental imagery in sighted and congenitally blind adults

In both congenitally blind and sighted adults, the production of mental images was associated with activation of visual cortex