PROCESSING AND TRANSMISSION OF VISUAL
IMPULSE
The receptor potential generated in the photoreceptors is transmitted by electrotonic
conduction (i.e., direct flow of electric current, and not as action potential) to other cells of the retina viz. horizontal cells, amacrine cells, and ganglion cells. However, the ganglion cells transmit the visual signals by means of action potential to the neurons of lateral geniculate body and the later to the primary visual cortex. The phenomenon of processing of visual impulse is very complicated. It is now clear that visual image is deciphered and analyzed in both serial and parallel fashion. Serial processing. The successive cells in the visual pathway starting from the photoreceptors to the
cells of lateral geniculate body are involved in increasingly complex analysis of image. This is called sequential or serial processing of visual information. Parallel processing. Two kinds of cells can be distinguished in the visual pathway starting from the ganglion cells of retina including neurons of the lateral geniculate body, striate cortex, and extrastriate cortex. These are large cells (magno or M cells) and small cells (parvo or P cells). There are strikinging differences between the sensitivity of M and P cells to stimulus features (Table 2.1).
Table 2.1. Differences in the sensitivity of M and P cells to stimulus features
Stimulus feature Sensitivity M cell :: P cell
Colour contrast No Yes
Luminance contrast Higher Lower
Spatial frequency Lower Higher
Temporal frequency Higher Lower
The visual pathway is now being considered to be made of two lanes: one made of the large cells is called magnocellular pathway and the other of small cells is called parvocellular pathway. These can be compared to two-lanes of a road. The M pathway and P pathway are involved in the parallel processing of the image i.e., analysis of different features of the
image.
IMPULSE
The receptor potential generated in the photoreceptors is transmitted by electrotonic
conduction (i.e., direct flow of electric current, and not as action potential) to other cells of the retina viz. horizontal cells, amacrine cells, and ganglion cells. However, the ganglion cells transmit the visual signals by means of action potential to the neurons of lateral geniculate body and the later to the primary visual cortex. The phenomenon of processing of visual impulse is very complicated. It is now clear that visual image is deciphered and analyzed in both serial and parallel fashion. Serial processing. The successive cells in the visual pathway starting from the photoreceptors to the
cells of lateral geniculate body are involved in increasingly complex analysis of image. This is called sequential or serial processing of visual information. Parallel processing. Two kinds of cells can be distinguished in the visual pathway starting from the ganglion cells of retina including neurons of the lateral geniculate body, striate cortex, and extrastriate cortex. These are large cells (magno or M cells) and small cells (parvo or P cells). There are strikinging differences between the sensitivity of M and P cells to stimulus features (Table 2.1).
Table 2.1. Differences in the sensitivity of M and P cells to stimulus features
Stimulus feature Sensitivity M cell :: P cell
Colour contrast No Yes
Luminance contrast Higher Lower
Spatial frequency Lower Higher
Temporal frequency Higher Lower
The visual pathway is now being considered to be made of two lanes: one made of the large cells is called magnocellular pathway and the other of small cells is called parvocellular pathway. These can be compared to two-lanes of a road. The M pathway and P pathway are involved in the parallel processing of the image i.e., analysis of different features of the
image.
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