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

A FFERENT B ASIS OF V ISUAL R ESPONSE P ROPERTIES IN A REA MT OF THE M ACAQUE . I. E FFECTS OF S TRIATE C ORTEX R EMOVAL Review of Rodman, H. R., Gross, C. G., and Albright, T. D. (1989) Louis Janse van Rensburg & Terence Tong

M IDDLE T EMPORAL A REA (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

B ACKGROUND ¢ 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

D EPENDENCE 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

E FFECTS OF S TRIATE C ORTEX L ESIONS

M ETHODS & M ATERIALS ¢ 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)

M ETHODS & M ATERIALS ¢ 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.

M ETHODS & M ATERIALS ¢ 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.

M ETHODS & M ATERIALS ¢ 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/ outside destroyed visual field).

R ESULTS ¢ 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

R ESULTS ¢ 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)

N O . 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)

R ESULTS ¢ 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.

R ESULTS ¢ 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 — of visual field.

R ESULTS ¢ 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 , x 2 = 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.

R ESULTS ¢ 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.

R ESULTS : ¢ Unilateral/bilateral comparison: ¢ Role of commisural/callosal inputs

R ESULTS : D IRECTION SELECTIVITY

R ESULTS : UNILATERAL C ASE Question: Significance of RF in lesion zone compared to RF on Midline?

R ESULTS : DIRECTION S ELECTIVITY ¢ No difference between normal/Striate cortex lesion — Possible role for MT as generating direction selectivity “de novo”

R ESULTS : D IRECTION & T UNING S ELECTIVITY ¢ No differences

R ESULTS : 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

E FFECTS OF S TRIATE C ORTEX C OOLING

M ATERIALS AND M ETHODS

R ESULTS

D ISCUSSION ¢ Afferent basis of residual visual responsiveness in MT — Dorsal LGN — MST and VIP — Spared portions of peripheral striate cortex — Tectopulvinar-MT path

D ISCUSSION ¢ 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

O RIGIN OF D IRECTIONAL SELECTIVITY IN MT ¢ Striate cortex unnecessary for directionally selective properties. — Direction selectivity and tuning not different within/ outside 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).

P OSSIBLE ROLE FOR MT IN B LINDSIGHT ¢ 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.