October 22, 2008 15:59 World Scientific Book - 9in x 6in BioElectronicVisionMartinsSousa

Characterization of the Neural Response 67

(a) Random dots. (b) White noise.

Fig. 3.10 Stimuli with spatial and temporal modulation.

In the current experimental apparatus, a computer monitor, with the

cathode ray tube (CRT) or thin-film transistor (TFT) technology, is used to

present the stimulus that is guided through an optical system to stimulate

the retina. Figure 3.11 displays a more complex stimulus, where each frame

is divided into small squares that, in the limit case, can be the image

picture elements, whose RGB intensities are driven by randomly sampling

a Gaussian distribution. In order to display the stimulus on a computer

monitor, the stimulus must be discretized in time. The stimulus in Fig. 3.11

is discretized, where ti+1 = ti +t so that t is the time bin width, that

establishes the sampling period of the stimuli, Ts. The images displayed

by modern computer monitors are digital, so each frame is composed of

a set of small picture elements (pixels), which correspond to an inherent

fine-grained spatial sampling.

The retina has a response to the light stimulus intensity spanning several

orders of magnitude, from a single photon to an influx of several millions of

photons per second. The retina, similarly to other sensory organs, performs

stimulus intensity compression, adapting to the mean level of the stimulus,

and senses only deviations from the stimulus mean following the Weber-

Fechner law (see Sec. 2.4.1). In order to simplify the study of the responses

of retinal ganglion cells, the stimuli can be described only in terms of their

var around their mean level. For a continuous-time stimulus this means

that:

1

T

T0

s(t)dt = 0 , (3.5)

where T is the stimulus time duration. For a discrete time stimulus used