Response to the length of moving visual stimuli of the brisk classes of ganglion cells in the cat retina

High speed coding for velocity by archerfish retinal ganglion cells

Background

Archerfish show very short behavioural latencies in response to falling prey. This raises the question, which response parameters of retinal ganglion cells to moving stimuli are best suited for fast coding of stimulus speed and direction.

Results

We compared stimulus reconstruction quality based on the ganglion cell response parameters latency, first interspike interval, and rate. For stimulus reconstruction of moving stimuli using latency was superior to using the other stimulus parameters. This was true for absolute latency, with respect to stimulus onset, as well as for relative latency, with respect to population response onset. Iteratively increasing the number of cells used for reconstruction decreased the calculated error close to zero.

Conclusions

Latency is the fastest response parameter available to the brain. Therefore, latency coding is best suited for high speed coding of moving objects. The quantitative data of this study are in good accordance with previously published behavioural response latencies.

The suppressive field of neurons in lateral geniculate nucleus

The responses of neurons in lateral geniculate nucleus (LGN) exhibit powerful suppressive phenomena such as contrast saturation, size tuning, and masking. These phenomena cannot be explained by the classical center-surround receptive field and have been ascribed to a variety of mechanisms, including feedback from cortex. We asked whether these phenomena might all be explained by a single mechanism, contrast gain control, which is inherited from retina and possibly strengthened in thalamus. We formalized an intuitive model of retinal contrast gain control that explicitly predicts gain as a function of local contrast. In the model, the output of the receptive field is divided by the output of a suppressive field, which computes the local root-mean-square contrast. The model provides good fits to LGN responses to a variety of stimuli; with a single set of parameters, it captures saturation, size tuning, and masking. It also correctly predicts that responses to small stimuli grow proportionally with contrast: were it not for the suppressive field, LGN responses would be linear. We characterized the suppressive field and found that it is similar in size to the surround of the classical receptive field (which is eight times larger than commonly estimated), it is not selective for stimulus orientation, and it responds to a wide range of frequencies, including very low spatial frequencies and high temporal frequencies. The latter property is hardly consistent with feedback from cortex. These measurements thoroughly describe the visual properties of contrast gain control in LGN and provide a parsimonious explanation for disparate suppressive phenomena.

Directional asymmetries in the length-response profiles of cells in the feline dorsal lateral geniculate nucleus

1. The visual cortex provides a major synaptic input to the dorsal lateral geniculate nucleus (dLGN). Cortical layer VI cells giving rise to this projection are strongly influenced by stimulus orientation, length and direction of motion. In the dLGN, a significant component of the strong length tuning exhibited by most cells follows from the corticofugal influence. We have now checked whether there are directional biases in geniculate cell responses, and whether such biases are influenced by stimulus length. 2. The responses of A-laminae dLGN cells were assessed by single-unit extracellular recording. Length preference was examined by plotting multihistogram length-tuning curves to moving bars of light of various length. 3. Over half of the cells tested (100/183) exhibited directional bias and in many cases, this bias was highly dependent on bar length, resulting in radically different length response profiles for the two directions of motion. These asymmetries are similar to those documented for cortical hypercomplex cells, but do not equate to any known facet of the centre-surround organization of dLGN cell receptive fields. 4. We suspected the directional biases followed from the influence of the corticofugal projection. To test this, we recorded from preparations where areas 17 and 18 of the visual cortex had been removed. Surprisingly, a similar proportion of cells exhibited directional biases after removal of the corticofugal input, suggesting that the biases are generated subcortically. 5. The widespread presence of systematic biases in the response profiles of dLGN cells further underlines the possibility that geniculate mechanisms may make a far greater contribution to visual processing than hitherto suspected.

Function of the Y optic nerve fibres in the cat: do they contribute to acuity and ability to discriminate fast motion?

1. A controlled pressure block has been applied to the optic nerve of the cat, sufficient to bring about degeneration of the axons of the large (Y) nerve fibres caudal to the block site. This degeneration has been monitored by means of implanted electrodes in optic nerve and tract which have shown a loss of the short-latency (t1) response 4-6 days after the block, and also by histological examination of the optic nerve. 2. Cats with one optic nerve blocked in this way have been used in behavioural experiments, one or other eye being covered during the tests. Tested via the blocked nerve, all cats with loss of only Y fibres could perform certain tests: the visual placing reaction, the blink reflex, the pupillary (light) reflex and simple manoeuvres such as walking a plank and jumping from table to floor. 3. When acuity was tested by means of the Mitchell jumping apparatus, cats with loss of only Y fibres showed the same acuity using either eye. This was true also of one cat in which many X fibres had also degenerated, as evidenced by a 55% loss of the medium-latency (t2) response, but in another cat with 90% loss of the t2 response acuity was reduced to about half-normal. 4. Ability to discriminate fast motion was tested by a modification of the Mitchell apparatus. All cats were able to discriminate the motion of an 11.5 deg spot up to a velocity of 6260 deg/s, whether using their normal eye or their affected eye. However, the loss of the Y fibres reduced the ability to discriminate fast motion, so that for any given level of contrast the velocity which could be discriminated was about two-thirds of the velocity discriminated using the normal eye. The ability of the cat to discriminate fast motion seems to be similar to that of the human. 5. These results suggest that there is no sharp restriction of function between the Y and X systems but instead considerable overlap. However, each system possesses specialized features giving it superiority in certain conditions.

A comparison of visual responses of cat lateral geniculate nucleus neurones with those of ganglion cells afferent to them

We compared visual responses of cat lateral geniculate nucleus (l.g.n.) neurones with those of retinal ganglion cells providing their afferent inputs. Quantitative studies were made on twenty such pairs; eight X on-centre, seven Y on-centre, two X off-centre and three Y off-centre pairs. Receptive field centre locations of cell pairs with correlated activities were very closely superimposed, having a mean centre displacement of 1.6 minutes of arc for X cells and 11 minutes of arc for Y cells. With flashed spots and annuli, responses of l.g.n. cells were almost always smaller than those of their retinal afferents, with peaks and troughs in ganglion cell responses being faithfully followed in the geniculate neurones. This is consistent with almost all impulses from the l.g.n. cell being triggered by the afferent feeding its centre. With spots of different sizes and contrasts, modulation of responses by l.g.n. inhibition was obvious, but effects were complex. With moving bright-bar stimuli, although response histograms were clearly reshaped to some extent in the l.g.n., peak firing rates under different stimulus conditions were often merely attenuated by a constant factor for most l.g.n. cells in comparison with their retinal inputs. For velocity tuning curves, a few cell pairs showed selective attenuation at high speeds, while others showed it at low speeds. All the latter group appeared to have more than one major excitatory afferent. These changes in velocity tuning occurred across the X/Y classification, so that differences in velocity preference of the X and Y systems is more blurred in the l.g.n. than in the retina.

Orientation bias of brisk-transient y-cells of the cat retina for drifting and alternating gratings

Brisk-transient ganglion cells of the cat's retina were examined for orientation bias using two different stimuli: drifting gratings and alternating gratings, both of fixed contrast (50%) and fixed temporal frequency (2 Hz). Some cells were strongly biassed for both stimuli, some were not biassed for either while still others were strongly biassed for only one or other stimulus. The preferred orientations for the two types of grating tended to be the same, on average, but substantial differences were not uncommon. A systematic preference for radially-oriented gratings was evident when the stimulus was drifting but there was an additional preference for tangentially-oriented gratings when the stimulus was alternating. Orientation bias for drifting gratings often extended over a broad range of spatial frequencies and was maximum near the resolution limit. For alternating gratings, bias was evident only at the highest spatial frequencies. Results indicate that the arrangement of receptive field components responsible for linear and nonlinear kinds of behaviour may sometimes possess different axes of symmetry.

Response to the velocity of moving visual stimuli of the brisk classes of ganglion cells in the cat retina

Extracellular recording of the responses of cat retinal ganglion cells to narrow moving bars revealed systematic response variations with changes in stimulus velocity. These response variations were studied by collecting peri-stimulus histograms from brisk-sustained (X) and brisk-transient (Y) ganglion cells as narrow elongated bars were moved backwards and forwards across their receptive fields. Velocity-response curves were produced from plots of the amplitude of the main peak of the histograms as a function of velocity. The shape of these curves was found to be reasonably constant for both classes of ganglion cells. For a given cell, the peak of the velocity-response curve shifted to both a higher response level and a higher velocity as the stimulus contrast was increased. Within both classes of cells there was a systematic shift in the velocity-response curve as a function of the size of the receptive field centre. For brisk-sustained cells this was seen as an increase in both the response and velocity at the peak for larger centre sizes, while for brisk-transient cells it was an increase in velocity at the peak with negligible change in response. When the velocity required to produce a small criterion response was determined, there were distinct differences between the two classes of cells. When plotted on a double-logarithmic scale as a function of centre size the brisk-sustained cells had a slope of 2.00 while brisk-transient cells had a slope of 1.20. Within the area centralis brisk-transient cells responded more readily at high velocities than brisk-sustained cells. This was not the case in the peripheral retina, where both cell classes responded about equally at high velocities.