Prepotentials preceding a neuronal action potential were recorded extracellularly in the lateral geniculate nucleus (LGN) of the macaque. Although prepotentials are found less frequently in the macaque than in the cat LGN, their electrical characteristics are similar, suggesting that they represent the arrival of impulses in a retinal afferent, as in the cat. The visual response properties of prepotentials and associated cells were similar under a variety of conditions, indicating that, apart from some response attenuation, little signal processing takes place in macaque LGN. A constant fraction of prepotentials above a threshold frequency gave rise to neuronal action potentials independent of the stimuli used, so that the frequency of cell action potentials was linearly related to the frequency of prepotentials. Since the maintained discharge rates of a cell and its prepotential always fell on the linear relation, the net responses of a cell and its prepotential to visual stimuli were approximately proportional to one another.

The responses of neurones in the lateral geniculate nucleus (LGN) were investigated in anaesthetised rhesus monkeys. A new classification for cells in the parvocellular layers (PCL) is proposed, based on their spectral response curve and their response to white stimuli: (A) narrow-band, short wavelength (NS) excited cells, activity suppressed by white stimuli; (B) wide-band, short-wavelength (WS) excited cells, excited by white stimuli; (C) wide-band, long-wavelength (WL) excited cells, (D) narrow-band, long-wavelength (NL) excited cells, activity suppressed by white stimuli; (E) light suppressed (LI) cells, activity suppressed by all wavelengths, usually with some concealed excitatory input at extreme short or long wavelengths. Responses to moving bars and to spots of various diameters (area response curves) were determined for various wavelengths. It was found that the receptive fields from which wavelength-dependent excitatory or suppressive effects could be elicited are concentrically superimposed. The spectral responsiveness of the excitatory inputs to individual cell types corresponds to the absorption curves of single cones (S-, M- or L-cone for NS, WS and WL cells respectively), the spectral distribution of the suppressive mechanisms of all cells was panchromatic and approximately fitted to a sum of all cones. The excitatory input to NL-cells cannot be related to any of the known cone absorption curves, and a simple (L-M) subtraction model is questioned. Neurones in the magnocellular layers (MCL) can be divided into on- and off-centre cells as in the cat's LGN and give qualitatively similar responses over the whole spectrum. In contrast to the tonic responses of PCL cells, MCL cells respond phasically to chromatic and white flashed spots, even with the smallest stimuli. Implications of these findings for colour processing in the LGN are discussed.

The receptive fields of LGN cells were investigated with stationary light and dark spot and annulus stimuli. Stimulus size and background intensity were varied while stimulus/background contrast was kept constant. The speed of dark adaptation vaired considerably from cell to cell. Dark adaptation made responses more sustained in all neurones and eliminated the oscillatory on-responses evoked under some conditions in the light-adapted cells. Dark adaptation led also to a disappearance of early phasic inhibition in on-responses, and increased response rise time and latency. The power of surround responses to inhibit centre responses decreased slightly at low levels of light adaptation in LGN cells but much less than in retinal ganglion cells. Some other traces of changing retinal surround effects also appeared inthe LGN on dark adaptation. For example, the functional size of receptive fields increased at low levels of illuminance as has been observed in retinal ganglion cells and the receptive fields as estimated from response peaks were larger than those estimated from sustained components.

Responses of macaque lateral geniculate nucleus (LGN) cells to stimuli of different incremental intensities and wavelength compositions were studied at different levels of light adaptation from scotopic to low photopic levels. Stimuli were large in comparison with receptive-field size. Human increment thresholds were measured for comparison. The strength of responses grew in many cells from threshold up to a saturation level as a logarithmic function of incremental intensity. More complex intensity-response functions were also obtained, particularly from parvocellular layer (PCL) cells, but the shape and slope of the intensity-response function changed as a function of adaptation level only with chromatic backgrounds. As a function of adaptation level, the intensity-response functions shifted along the logarithmic abscissa but not sufficiently for a complete contrast constancy. Thus responses to any constant contrast became smaller when adaptation level decreased. The change from cone to rod responses, when possible, took place without noticeable change in shape of intensity-response functions, and much of the adaptive shift of the functions could be attributed to the change-over between rods and cones. Differences between different cells in light adaptation and dark-adapted sensitivity were large, mostly because of variation in the strength of rod input. The strongest excitatory rod inputs were found in PCL cells activated by short-wavelength light, so that the highest sensitivity at low levels of illumination occurred in blue- and blue-green-sensitive cells. The lowest increment thresholds based on cones matched the thresholds of macaque cone late receptor potentials recorded by Boynton and Whitten (3). They were also similar to human cone thresholds measured psychophysically but only for small stimulus sizes that may approximate the size of the receptive-field centers. Human sensitivity was much higher when measured with large stimulus sizes, indicating integration at post-geniculate neural levels. Light adaptation, as evaluated with respect to contrast constancy and Weber law behavior, was similarly incomplete in monkey single cells and human perception. A few cat LGN cells were studied in a control experiment; results agreed with previous findings. The light adaptation of cat cels was more complete and sensitivity higher than those observed under comparable conditions in macaque single cells and human. The maintained activity level of cells was little affected by the intensity of steady backgrounds. Thus, the steady-state hyper-polarisation of receptors was not transmitted to LGN cells.

The responses to moving stimuli of single cells in the parvo- and magnocellular layers (PCL and MCL) of the macaque lateral geniculate nucleus (LGN) have been studied. PCL cells respond with a monophasic increase or decrease in firing when a bar passes across the receptive field, according to the wavelength composition of the stimulus. MCL cells respond with a biphasic sequence of excitation and suppression or vice versa dependent on whether a cell is on-centre or off-centre and on stimulus contrast direction. With large stimuli, PCL cells respond as long as the stimulus covers the receptive field while MCL cells respond only at the contrast borders. MCL cell responses are maximal with bars just long enough to cover the field centre, while PCL cell responses show a variable relation with bar length, depending on stimulus wavelength and receptive field structure. PCL cells show broad velocity tuning while at least some MCL cells were more sharply tuned. Many cells in the macaque LGN show weak orientation or direction preference.