126 Neural Aspects of Vision \1 : 2 



of the eighth retinal layer. The k and / types are lateral amacrine cells 

 synapsing with the other bipolars of the same layer. 



The cell bodies of the innermost layer of neurons in the retina are 

 called ganglion cells. Three identified types are used in Talbot's model. 

 The largest are the m cells which synapse with fibers from d, e, and f 

 bipolars. The middle-sized p cells also synapse, albeit in a different 

 fashion, with fibers from d, e, and/. Finally, the smallest cell bodies, 

 labeled s, synapse only with one h bipolar. 



Any attempt to assign a function to each of these elements is guess- 

 work. In this proposed model, it is assumed that the neurons have a 

 natural firing period even when they are not stimulated by the rods and 

 cones. The cell bodies of the latter also produce the spike action- 

 potentials even when the rods and cones are not exposed to light. As 

 discussed in Chapters 4 and 5, a network of neurons, such as exist in the 

 retina, can add, subtract, multiply, and divide in a fashion somewhat 

 similar to an electronic digital computer. The action potentials which 

 go to the brain may be a complex function of the incident light. 



Talbot states that since the h-s pathway is the only one which does 

 not become more diffuse as it proceeds toward the central nervous 

 system, it can carry the detail necessary for acuity perception. There- 

 fore, he assigns it the role of black and white vision under photopic 

 conditions. Because the d-m pathway represents the largest, easiest to 

 excite cells, and because it is connected to the rods p, it must carry the 

 scotopic white information. 



To produce antagonistic effects, the responses of the three receptors 

 could be combined at successive neurons as illustrated in Figure 2. 

 The responses of p and 8 are added at d to give a blue response B. The 

 spikes of i (as reproduced by f) and of d are added at p to give a red 

 response. Because the B and G fibers synapse very close to the side of the 

 m and p cells respectively, their spikes are assumed to be inhibitory, 

 that is, they slow down the natural firing rate. Yellow, made up from 

 G and R, would then accelerate m, whereas R would accelerate p. Thus, 

 Talbot has an antagonist theory whereby white and black are antago- 

 nistic at the ganglion cell s, blue and yellow at the ganglion cell m, and 

 green and red at the ganglion cell p. 



In order to decrease a firing rate, m and p must have a normal firing 

 rate regulated by feedback loops set up through the k and / bipolars. 

 Similarly, in order to suppress the effects of glare and scattering within 

 the eye and to decrease firing during prolonged stimulation, fibers such 

 as those of the i type cell must be present. Neural sharpening can also 

 be produced by i, k, and / cells. 



Anatomically, Talbot's model is quite successful in assigning a role 

 to almost all of the histologically distinct neuron types in the retina. 



