A FFERENT B ASIS OF V ISUAL R ESPONSE P ROPERTIES IN A REA MT OF THE - - PowerPoint PPT Presentation

AFFERENT BASIS OF VISUAL RESPONSE PROPERTIES IN AREA MT OF THE MACAQUE. I. EFFECTS

OF STRIATE CORTEX REMOVAL

Review of Rodman, H. R., Gross, C. G., and Albright,

• T. D. (1989)

Louis Janse van Rensburg & Terence Tong

MIDDLE TEMPORAL AREA (MT)

¢ Posterior Bank of the Superior Temporal Sulcus

Taken from http://thebrain.mcgill.ca/flash/a/a_02/a_02_cr/a_02_cr_vis/a_02_cr_vis.html

BACKGROUND

¢ MT neurons selective for both direction and

speed of stimulus motion

¢ Cells with similar preferred direction are in

columns

¢ Lesion studies show disruption of thresholds for

detection and discrimination of visual motion

¢ Connections — Striate Cortex — V2, V3 — MST, FST and VIP — V4, and possibly V3A and PO — Inferior and Lateral Pulvinar — Claustrum, pons and superior colliculus

DEPENDENCE OF MT

¢ Striate Cortex — Major part of ascending input comes from Striate

cortex, V2, V3

— Loss of visual responsiveness following removal of

striate input

— Antidromic experiments ¢ Other Regions — Superior Colliculus — Tectopulvinar-MT path — Superior temporal polysensory area

EFFECTS OF STRIATE CORTEX LESIONS

METHODS & MATERIALS

¢ Subjects and Striate Cortex lesions: — 3 Male Macaca fascicularis (no. 542, 555, 561)

¢ Partial unilateral striate cortex ablation (542) ¢ Partial bilateral striate cortex ablation (555) ¢ Total bilateral striate cortex ablation (561) ¢ 5-6 week recovery ¢ Animals were restrained and anesthetized during all

surgeries (Atropine, Ketamine, Nitrous oxide + Fluothane, Pavulon and Valium)

METHODS & MATERIALS

¢ Preparation for recording — Prior to Striate Cortex Lesion Surgeries:

¢ Stereotaxically positioned 5cm diameter stainless steel

recording chamber implanted and fixated with screws and dental acrylic over midline.

¢ 1 week before lesion surgeries

— Prior to recording

¢ Pupils dilated (Cyclogyl) ¢ Corneas covered with contact lenses to ensure image

fixation on tangent screen at 57 cm.

METHODS & MATERIALS

¢ Recording Procedure and Visual Stimulation — MT Single/Multiunit recordings using T.

microelectrodes

— Single/multiunit sites tested for:

¢ Visual responsiveness ¢ Direction selectivity ¢ This refers to relative strength of responses to preferred

• vs. anti-preferred directions

¢ 0 = no direction preference, 5 = strong preference ¢ Broadness of direction tuning/sharpness/strength of

direction selectivity

¢ i.e. how selective that single/multiunit site is for angular

directions ranging further away from optimal direction

¢ 0 = no selectivity, 5 = strong selectivity. ¢ Binocularity of unit sites ¢ Unit Responses compared for ipsi/contralateral eye input.

METHODS & MATERIALS

¢ cont. — RF’s corresponding to MT unit sites mapped:

¢ Using smallest stimulus capable of evoking cell responses ¢ Determined borders by assessing where in the visual field

responses stopped.

— Computer-controlled stimulus display varying stimulus:

¢ Size (0.5-1 degree width x 3-20 degree length) ¢ Speed (2 -64 degrees/second) ¢ Angular Direction 8 or 16 different directions of bar/slit

stimuli movement (always perpendicular to length of the bar)

¢ “optimal with hand-testing” (Blind-sight implication

discussed)

— Histology: perfusion, staining & 50 nanometer slices — Also assessed LGN degradation corresponding to Striate

Lesioning and calculated:

¢ 1 degree error in estimated visual field representation at 10

degree eccentricities, and (for no. 542 only) 5-10 degree error at 40 degree eccentricity. (IMPORTANT: Comparison RF within/

• utside destroyed visual field).

RESULTS

¢ Based on 269 MT sites (165 single, 104 multi) — Histologically determined to be within

myeloarchitectonic borders of MT

— 3 categories of MT unit RF correspondence to Visual

Field defect

¢ RF almost entirely within Visual field defect (RF area =

%80-%100 within defect)

¢ RF partially within (%20-80%) ¢ RF outside

RESULTS

¢ Histological Verification of Topographic/Visual

Map Defect

— Assessed Retrograde Degeneration of LGN and

corresponding Striate Cortex Ablation (accuracy/ control measure)

— Case no. 542 Unilateral lesion

¢ Left Striate cortex lesion: right hemi-field defect ¢ 10 degrees above midline at foveal region ¢ 20 degrees above and below horizontal meridian ¢ 45 degrees most peripheral (northeast/top right in visual

field)

• NO. 542

Case no. 542 Unilateral lesion

• Left Striate cortex lesion:

right hemi-field defect

• 10 degrees above midline at

foveal region

• 20 degrees above and below

horizontal meridian

• 45 degrees most peripheral

(northeast/top right in visual field)

RESULTS

¢ Case no. 555 (partial

Bilateral lesion)

— Symmetrical — Dorso-medial edge along

lunate sulcus spared

— Slight invasion of

posterior portion of Calcarine Sulcus

— Limited damage to V2 — Circular defect: 4

degrees extension upper field, 6 degrees into the lower field and 7 degrees along horizontal meridian.

RESULTS

¢ Case no. 561 —

Total lesioning of Striate except for small “tag” of anterior-most calcarine sulcus.

—

Extended past Lunate Sulcus into V2 (dorsally)

—

Ventral Extrastriate most sever in right hemisphere (V2 and V3 involvement), but similar damage to left hemisphere also.

—

Some extension into white matter above upper calcarine sulcus bank

—

Possible damage to posterior MT (no unit response) but anterior still responded despite damage to STP and gray matter of MT. Anterior recordings used in data analysis.

—

Visual defect total for 60degrees

• f visual field.

RESULTS

¢ Overall Recording Quality (Computer and Auditory — MT unit RFs that feel within lesioned zone:

¢ MT unit response unlike Normal MT response- weak bursty

spontaneous activity (sounded “injured”)

¢ Single units hard to isolate

¢ Responsiveness (strong, weak, no response) — Following lesions, 66% of isolated units were still responsive

(for all lesion cases, i.e. no lesion-case dependent differences in responsiveness, x2 = 6, df =1, p>,2)

¢ However, only 5% “strong” (RFs within defect zone) ¢ Units with RFs outside lesion zone, gave strong responses, but not as

many as in normal MTs.

— All RF categories in lesion cases sig. differed to normal in

terms of proportion of strong, weak and no response (chi square tests, all p values < .02 ).

— No significant differences found between lesion groups in

terms of category responses (all p values > .2). Therefore amalgamated response data for all categories.

RESULTS

¢ What does this mean? — Taken as evidence to reject the idea that intact

striate cortex could be determining MT response

— No difference between response in total and partial

bilateral lesions.

RESULTS:

¢ Unilateral/bilateral comparison: ¢ Role of commisural/callosal

inputs

RESULTS: DIRECTION SELECTIVITY

RESULTS: UNILATERAL CASE

Question: Significance of RF in lesion zone compared to RF on Midline?

RESULTS: DIRECTION SELECTIVITY

¢ No difference between

normal/Striate cortex lesion

— Possible role for MT as

generating direction selectivity “de novo”

RESULTS: DIRECTION & TUNING SELECTIVITY

¢ No differences

RESULTS: OTHER FINDINGS

¢ Binocularity — 40 MT unit RF response did not differ substantially

between eyes

¢ Few responses better for either contra/ispsi but no strong

monocularity

¢ RF field size as function of eccentricity for single

units: Regression analysis (no sig difference between normal/lesion cases).

¢ Speed Selectivity: — Direction dependent

¢ Not explored further

EFFECTS OF STRIATE CORTEX COOLING

MATERIALS AND METHODS

RESULTS

DISCUSSION

¢ Afferent basis of residual visual responsiveness in

MT

— Dorsal LGN — MST and VIP — Spared portions of peripheral striate cortex — Tectopulvinar-MT path

DISCUSSION

¢ Contribution of striate cortex to response properties — Heterogeneous population of neurons dependent on striate

cortex for responsiveness to light

— Responsiveness not due to a recovery process — Effects outside the “lesion zone” — Callosal connections to responsiveness along the vertical

meridian representation in MT

— Marked shift or absence of selectivity — Direct or Indirect

ORIGIN OF DIRECTIONAL SELECTIVITY IN MT

¢ Striate cortex unnecessary for directionally

selective properties.

— Direction selectivity and tuning not different within/

• utside defect zone. (applies to control comparison

also)

— “Instrinsic circuitry” not striate dependent — Motter et al. (1987): MT generates direction

selectivity from nonselective inputs (Ascending and descending)

— BUT: doesn’t eliminate Striate role in directional

selectivity as 1/3 were no longer responding in the lesion cases. (Movshon and Newsome, 1984).

POSSIBLE ROLE FOR MT IN BLINDSIGHT

¢ Monkeys and humans can discern velocity and

direction.

¢ So far MT is the only area responsible for

generation of directional selectivity de novo in the absence of Striate input.

¢ Behavioral dysfunction: lowered gain can result

in inability to use information about velocity.