Impulse response of an S-cone pathway in the aging visual system

Abstract

Age-related changes in the temporal properties of an S-cone pathway were characterized by the psychophysical impulse-response function (IRF). Participants included 49 color-normal observers ranging in age from 16.8 to 86.3 years. A double-pulse method was used to measure the IRF with S-cone modulation at constant luminance. Stimuli were presented as a Gaussian patch (+/-1SD = 2.3 degrees ) in one of four quadrants around a central fixation cross on a CRT screen. The test stimulus was modulated from the equal-energy white of the background toward the short-wave spectrum locus. Each of the two pulses (6.67 ms) was separated by an interstimulus interval (ISI) from 20 to 720 ms. Chromatic detection thresholds were determined by a four-alternative forced-choice method with staircases for each ISI in one session. IRFs were calculated from the threshold data using a model with four parameters of an exponentially damped sine wave. S-cone IRFs have only an excitatory phase and a much longer time course compared with IRFs for luminance modulation measured with the same apparatus. The results demonstrated significant age-related losses in IRF amplitude, but the latency (time to peak) of the IRF was stable with age.

Fig. 1

Color angle required for a match of complementary colors rotated with respect to a white point (metameric with equal-energy white) when superimposed on a 420 nm adapting field.

Fig. 2

Left panel shows contrast threshold in S-cone trolands as a function of ISI for an older observer (78.3 years). Threshold data were double pulses separated by varying ISIs. Error bars denote ±1 SEM (standard error of the means). The solid curve in the right panel illustrates the impulse-response function fitted to the threshold data. The dotted curve denotes the responses based on the sum of 3 successive frames. The summed curve was used for model fitting. The solid curve shows the calculated impulse-response function for a single frame.

Fig. 3

S-cone IRFs of typical younger and older observers. Black and gray solid curves denote IRFs of younger observers (29.4 and 19.8 years, respectively). Black and gray dotted curves denote IRFs of older observers (74.3 and 73.3 years, respectively).

Fig. 4

Peak amplitude of the S-cone impulse response as a function of age. Solid line denotes the least-squares linear regression line for all observers except the two denoted by the × (for IOL or glaucoma patients). Open circles and squares denote those older observers with shorter and longer durations (as illustrated in Fig. 5), respectively.

Fig. 5

Duration of the S-cone impulse response (defined by 5% of the peak) as a function of age. The solid line denotes the least-squares linear regression for all observers except the two denoted by the × (IOL and glaucoma patient). Open circles and squares denote older observers with shorter and longer IRF durations, respectively.

Fig. 6

S-cone IRF duration plotted as a function of peak amplitude. Open and closed circles denote observers ≤70 or >70 years, respectively. The solid line denotes the least-squares linear regression line based on all observers except the two denoted by the × (IOL and glaucoma patients).

Fig. 7

Peak amplitude of the impulse response for luminance modulation plotted as a function of peak amplitude for S-cone modulation. Open and closed circles denote observers ≤70 or >70 years, respectively. The solid line denotes the least-squares linear regression line based on all observers except for the × (glaucoma patient).

Fig. 8

Duration of the impulse response for luminance modulation (first phase) plotted as a function of duration for S-cone modulation (5% of peak). Open and closed circles denote observers ≤70 or >70 years, respectively. The solid line denotes the least-squares linear regression line based on all observers except for the × (glaucoma patient).

Fig. 9

Left panel shows contrast sensitivity as a function of temporal frequency for an S-cone pathway of theoretical 20-year-old (black curve) and 80-year-old (gray curve) observers. The same panel shows tCSFs measured with luminance modulation (from Ref. 2): dotted, dotted–dashed, and dashed curves denote the tCSF for theoretical 20-year old and 80 year-old observers with normal and little secondary (inhibitory) phase, respectively. Right panel shows the theoretical IRFs used to generate these tCSFs calculated from the model equation. Curves are plotted as in the left panel.