6.4

• This can be determined by asking subjects to judge the relative brightness of different wavelengths of light shone on the periphery of the retina at an intensity too low to activate the few peripheral cones located there.

  scotopic spectral sensitivity curve
• True or False, Under photopic conditions, notice that the visual system is maximally sensitive to wavelengths of about 560 nanometers; thus, under photopic conditions, a light at 560 nanometers would have to be much more intense than one at 500 nanometers to be seen as equally bright.

  False, a light at 500 nanometers would have to be much more intense than one at 560 nanometers to be seen as equally bright.
• True or False Under scotopic conditions, the visual system is maximally sensitive to wavelengths of about 580 nanometers; thus, under scotopic conditions, a light of 560 nanometers would have to be much more intense than one at 500 nanometers to be seen as equally bright.

  False, wavelengths of about 500 nanometers
• The Purkinje Effect was pioneered by?

  Jan Purkinje
• Although we are not aware of it, the eyes continually scan the visual field, and our visual perception at any instant is a summation of recent visual information. It is because of this temporal integration that the world does not vanish momentarily each time we blink.
• Involuntary fixational eye movements are of three kinds:

  1. Tremor 2. Drifts 3. Saccades
• True or False, eye movements enable us to see during fixation by keeping the images moving on the retina.

  True
• Visual transduction is the conversion of light to neural signals by the visual receptors.
• When rhodopsin receptors are bleached by light, the resulting cascade of intracellular chemical events closes the sodium channels, hyperpolarizes the rods, and reduces the release of glutamate.
• When rods are in darkness, their sodium channels are partially open, thus keeping the rods slightly depolarized and allowing a steady flow of excitatory glutamate neurotransmitter molecules to emanate from them.
• Transduction of light by rods exemplifies an important point:

  Signals are often transmitted through neural systems by decreases in activity.
• This conduct signals from each retina to the primary visual cortex (also known as striate cortex or V1) via the lateral geniculate nuclei of the thalamus.

  retina-geniculate-striate pathways
• Most of the lateral geniculate neurons that project to the primary visual cortex terminate in the lower part of cortical layer IV, producing a characteristic stripe, or striation, when viewed in cross section— hence, primary visual cortex is often referred to as striate cortex.
• The retina-geniculate-striate system is retinotopic; each level of the system is organized like a map of the retina.
• This particularly responsive to color, fine pattern details, and stationary or slowly moving objects and cones provide the majority of the input to this layer

  
  

  parvocellular layers (or P layers)
• These cells are particularly responsive to movement and rods provide the majority of the input to this.

  Magnocellular Cells (M Layers)
• The place where two different areas of a visual image meet.

  Visual Edge
• These are nonexistent stripes of brightness and darkness running adjacent to the edges which enhance the contrast at each edge and make the edge easier to see.

  Mach Bands
• An area of the visual field within which it is possible for a visual stimulus to influence the firing of that neuron.

  Receptive Field
• Four commonalties recorded from retinal ganglion cells, lateral geniculate nuclei, and lower layer IV striate neurons:

  1. receptive fields in the foveal area of the retina were smaller than those at the periphery
  2. All the neurons had receptive fields that were circular.
  3. All the neurons were monocular.
  4. It had receptive fields that comprised an excitatory area and an inhibitory area separated by a circular boundary.