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Spectral sensitivity in hemianopic macaque monkeys.

A Cowey1, P Stoerig

  • 1University of Oxford, Department of Experimental Psychology, UK. Alan.Cowey@psy.ox.ac.uk

The European Journal of Neuroscience
|May 21, 1999
PubMed
Summary
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Following striate cortex removal in monkeys, visual field sensitivity was reduced but functional. Both rod and cone mechanisms contribute to residual vision, challenging the idea that only rods mediate vision after striate cortex damage.

Area of Science:

  • Neuroscience
  • Vision Science

Background:

  • The striate cortex (V1) is traditionally considered essential for conscious visual perception.
  • Blindsight, a phenomenon in patients with V1 damage, suggests residual visual capabilities in the absence of awareness.

Purpose of the Study:

  • To investigate visual sensitivity in macaque monkeys following unilateral striate cortex ablation.
  • To determine the contribution of rod and cone photoreceptor mechanisms to residual vision in the hemianopic field.

Main Methods:

  • Monkeys with prior left striate cortex removal and a normal control underwent visual testing.
  • Increment threshold sensitivity was measured for targets at eccentricities of 20-26 degrees in both hemifields.
  • Testing was performed under varying light adaptation levels (scotopic, mesopic, photopic).

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Main Results:

  • Hemianopic visual fields showed reduced sensitivity (up to 0.5 log units) for chromatic stimuli compared to normal fields.
  • A progressive shift in sensitivity favoring longer wavelengths (red) over shorter wavelengths (blue, green) occurred with increasing light adaptation.
  • This wavelength-dependent sensitivity change, akin to the Purkinje shift, was observed in both normal and hemianopic fields.

Conclusions:

  • Residual visual sensitivity exists in hemianopic fields after striate cortex removal in monkeys.
  • Both rod and cone photoreceptor systems contribute to this residual vision.
  • The findings indicate that visual processing beyond the striate cortex involves functional rod and cone pathways.