Receptive field properties of optic tract fibres from on-center sustained and transient cells in a tree shrew ( Tupaia chinensis)

Single unit recordings of on-center cells were made from optic tract fibres of the cone dominated retina of the primitive primate Tupaia chinensis. The characteristics of the responses to long duration (square-wave) pulses presented to the uniform centers of the dark-adapted receptive fields were used to divide the whole population into two groups: sustained and transient cells. Sustained units had relatively small centers and were concentrated near the area centralis; transient units had larger centers, which increased in size with increasing eccentricity. The spatial sensitivity gradient of the centre mechanism of sustained cells was significantly larger than that of transient cells. The absolute sensitivity of both types of units (quantum to spike ratio) appeared to be in the same range and did not correlate with the size of the receptive field center, nor with the maintained activity in the dark. The burst duration of the response to 10 msec light pulses was significantly shorter for sustained cells than for transient cells in the dark. Light adaptation was coupled in both types of cells with an increase in the quantum to spike ratio (QSR), and with a shortening of the burst duration; the difference in burst duration between the two types disappeared with light adaptation. Light adaptation (by diffuse fields) was also coupled with a regularly increasing maintained activity in sustained cells and an unchanged maintained activity in transient cells. In transient cells the starting level of light adaptation ( I t ) was inversely proportional to the uniform center area. In sustained cells this correlation was absent, but in these units I t was negatively correlated. with the slope of the curves relating maintained activity to adapting intensity level. The latter observation was interpreted as a positive correlation with the strength of the surround mechanism. Additionally, in sustained cells there was a strong negative correlation ( r = −0.9) between the maintained activity in the dark and I t . A slight positive correlation was present in transient cells. With center adapting spots and annular stimulation it was possible to obtain isolated off-responses in sustained cells. This was impossible in transient cells. The above-mentioned results strengthen the idea that center and surround mechanisms overlap completely in transient cells and are spatially better separated in sustained cells. Optic tract units of Tupaia began to adapt at a level of 10–10 3 td. a