Temporal Properties of Vision

Our visual perception arises from the interpretation of light information, which varies in space, wavelength, and time. It is the latter of these attributes that is explored in this chapter. Subjectively, the world appears to be stable despite continuous changes in the visual scene. How does the visual system respond to and interpret light variations that occur as a function of time?

The duration of a light affects both the ease with which we can see it, as well as its subjective appearance. This chapter emphasizes the first of these issues, that is, how sensitive we are to temporal variations in light and which factors influence our sensitivity. Temporal sensitivity cannot be studied in isolation because other stimulus attributes, such as spatial properties, chromaticity, background features and surround characteristics, all influence our ability to detect temporal variations. In the natural world, most temporal variation occurs through image motion. This may arise from motion of the observer, the eyes, or the object itself. Motion is a special form of temporal variation in which the change with time is associated with a change in spatial position.

This chapter summarizes a number of basic phenomena that describe the sensitivity of our visual system to temporal information. The application of these phenomena to the clinical study of abnormalities of visual processing, such as in disease, is also discussed.

Temporal summation and the critical duration

To detect the presence of something in the visual world, it must be present for a finite period of time. Although a single quantum of light may be sufficient to generate a neural response, multiple quanta are generally required during a short period before the light is reliably seen, a property known as temporal summation. In the human visual system, temporal summation occurs for durations of approximately 40 to 100 milliseconds, depending on the spatial and temporal properties of the object and its background, the adaptation level, and the eccentricity of the stimulus. The maximum time over which temporal summation can occur is the critical duration.

Let's say we wish to determine how long a light needs to be presented on a dark background to be visible. In general terms, a more intense light does not need to be presented for as long as a less intense one to reach threshold visibility. The relationship between the luminance of the light and the duration of its exposure to reach visibility is linear over a limited range. Provided that the light pulse is shorter than the critical duration, it will be at threshold when the product of its duration and its intensity equals a constant. The formula that describes this time–intensity reciprocity is Bloch's law :

Bt=K $B t = K$

where: B = Luminance of the light, t = Duration, K = A constant value.

Bloch's law is shown schematically in Figure 37.1A . When stimulus intensity and duration are plotted on log-log coordinates, as in Figure 37.1A , Bloch's law describes a line with a slope of −1. When the critical duration is reached, the threshold intensity versus duration function is described by a horizontal line; that is, a constant intensity is required to reach threshold. Bloch's law has been shown to be generally valid for a wide range of stimulus and background conditions, including both foveal and peripheral viewing. Once the duration of the stimulus exceeds the critical duration, the luminance required for it to reach visibility is classically considered to be constant.

Figure 37.1, ( A ) Schematic of the idealized relationship between threshold light intensity and the duration for it to reach visibility. For durations less than the critical duration, the threshold intensity is linearly related to duration, as described by Bloch's law. ( B ) The relationship between threshold light intensity and duration for flickering grating stimuli for two observers as measured by Gorea & Tyler.

The preceding discussion assumes that whenever the observer's threshold is exceeded, he or she will respond accurately to the stimulus. This predicts an abrupt and idealized transition between the two curves, as depicted in Figure 37.1A . In the real world, both visual stimuli and the physiologic mechanisms that we use to detect them are subject to random fluctuations in response. We may consider the length of the stimulus presentation to be divided into a number of discrete time intervals. The signal is detected when the response exceeds threshold in at least one interval and the probability of detection in each interval is considered independent. This description of the probabilistic nature of visual detection is known as probability summation over time . The concept of probability summation is included in many models of temporal visual processing and is thought to be at least partly responsible for the less-than-abrupt transition between the region of temporal summation and constant intensity that occurs under some experimental conditions. One experimental situation in which this arises is when threshold contrast is measured as a function of signal duration for sinusoidal grating stimuli. This is illustrated in Figure 37.1B . In Figure 37.1B the upper curve shows threshold duration data for a 0.8 cycle per degree grating, and the lower curve shows those for an 8 cycle per degree grating. The upper curve conforms well to the schematic diagram that is illustrated in Figure 37.1A . For the lower curve depicting results for the 8 cycle per degree grating, however, a gradual transition is observed. In this latter case the actual critical duration (traditionally the point of intercept of the two slopes in Fig. 37.1A ) is somewhat difficult to define. Data similar to those shown in Figure 37.1B , have been interpreted to indicate that the critical duration increases with increasing spatial frequency. Gorea & Tyler use an alternative form of analysis that includes the effects of probability summation; they conclude that the critical duration is minimally affected by spatial frequency.

Factors affecting the critical duration

The time interval defined by the critical duration depends on properties of both the stimulus and the background. The critical duration has been shown to vary with light adaptation level; that is, with brighter backgrounds, the critical duration decreases. Conversely, with dark adaptation, the critical duration increases. Unless dark adapted, the size of the stimulus also affects the critical duration, with larger stimuli having a decreased critical duration. Retinal eccentricity also influences the critical duration, as does the visual task. Temporal summation is also affected by the spectral composition (wavelength or color) of the light stimuli, with isolated chromatic stimuli having longer temporal integration than achromatic (luminance) stimuli. For colored lights, the critical duration decreases with increased chromatic saturation of the background, similar to the decrease in critical duration with increased luminance for achromatic stimuli. Figure 37.2 demonstrates how chromaticity and retinal eccentricity markedly alter the sensitivity to temporal pulses and the critical duration.

Figure 37.2, Log contrast sensitivity versus pulse duration for equiluminant chromatic pulses. Data are shown for two observers (left and right panels). The upper panels show performance for “red-green” modulation (long-wavelength sensitive [L] and middle-wavelength sensitive [M] cone modulation); whereas the lower panels show data for short wavelength modulation.

Temporal sensitivity to periodic stimuli

The preceding section has considered how the human visual system responds to aperiodic stimuli (for example, a single pulse of light). We will now consider how the visual system responds to periodic stimuli (repeatedly flickering stimuli). Most research in this area has been directed to explore the following questions: (1) what is the fastest flicker rate that can be detected by the human visual system (the critical flicker fusion frequency); and (2) what factors influence sensitivity to flicker slower than this critical rate?

Critical flicker fusion frequency

When a light is turned on and off repeatedly in rapid succession, the light appears to flicker, provided the on and off intervals are greater than some finite time interval. If the lights are flickered fast enough, we perceive the flashes as a single fused light rather than a series of flashes. In simple terms, when the perception of fusion occurs, we have reached the limit of the temporal-resolving ability of our visual system. The transition from the perception of flicker to that of fusion occurs over a range of temporal frequencies; the boundary between the two is called the critical flicker fusion (CFF) frequency. The value of the CFF varies, depending on a large number of both stimulus and observer characteristics. Some of the important factors that influence the CFF ( Box 37.1 ) are discussed in the following sections.

Box 37.1

Critical flicker frequency (CFF)

The critical flicker frequency (CFF) describes the fastest rate that a stimulus can flicker and just be perceived as a flickering rather than stable.
While the CFF is dependent on the temporal resolution of visual neurons, it is also considered to be a measure of conscious visual awareness because, at CFF threshold, an identical flickering stimulus varies in percept from flickering to stable. Functional magnetic resonance imaging demonstrates involvement of the frontal and parietal cortex in the conscious perception of flicker.
CFF has been utilized as a measure of conscious visual awareness in a wide range of pharmacological and psychological research studies.
Perimetric tests of CFF have also been developed with most research directed towards visual field assessment in glaucoma.

Effect of stimulus luminance on CFF

In general, the CFF increases as the luminance of the flashing stimulus increases. This relationship is known as the Ferry–Porter law, which states that CFF increases as a linear function of log luminance. The Ferry–Porter law is valid for a wide range of stimulus conditions and is illustrated in Figure 37.3 . The lower curves ( solid lines and symbols ) show data collected in the fovea, and the upper curves show data collected at 35 degrees eccentricity. For both locations the upper curves are for smaller targets (0.05 degree foveally and 0.5 degree eccentrically), and the lower curves are for larger targets (0.5 degree foveally and 5.7 degrees eccentrically). Figure 37.3 demonstrates several interesting observations about the relationship between CFF and luminance. First, the Ferry–Porter law is upheld despite changes in stimulus size. Second, the linear relationship between log luminance and CFF is present for both central and peripheral viewing, although the slope of this relationship increases in the periphery, implying faster processing. The Ferry–Porter law holds not only for spot targets but also for grating stimuli. For scotopic luminance levels, at which rods mediate detection, CFF decreases substantially to approximately 20 Hz and no longer obeys the Ferry–Porter law.

Figure 37.3, Photopic critical flicker fusion (CFF) versus intensity functions measured in the fovea (solid symbols and lines) and at 35 degrees eccentricity (dotted and dashed lines) from Tyler & Hamer. (Reproduced with permission from Tyler & Hamer 1990. 24 ). The test stimulus was 660 nm presented on a equiluminant white surround. Foveal test stimuli were 0.5 degree (large filled circles) and 0.05 degree (small filled circles). Eccentric stimuli were 5.7 degrees (dashed lines) and 0.5 degree (dotted lines). Note the adherence to the Ferry–Porter law for all eccentricities tested. ECC, Eccentricity.

Effect of stimulus chromaticity on CFF

The linear relationship between CFF and log luminance, as described by the Ferry–Porter law, is also valid for purely chromatic stimuli. However, the slope of the relationship has been shown to vary with stimulus wavelength. This relationship is demonstrated in Figure 37.4 , which shows CFF versus illuminance functions derived from four separate studies. In all four studies the foveal CFF illuminance functions are well fit by Ferry–Porter lines, and in all cases the functions for green (middle wavelength) lights were found to be steeper than those for red (long wavelength) lights. The steeper slope for green stimuli has been interpreted as evidence supporting the green cone pathways being inherently faster than the red cone pathways for the transmission of information near the CFF. The CFF is lowest for blue stimuli detected by the short-wavelength pathways.

Figure 37.4, Critical flicker fusion (CFF) versus illuminance functions for red and green flicker measured by Hamer & Tyler. (Reproduced with permission from Hamer & Tyler 1992. 28 ). Note that all data are well fit by the Ferry–Porter law, with green lights having a steeper slope than red. ( A ) Data for one subject from Hamer & Tyler. 28 The green light function is steeper by 17 percent. ( B ) CFF data from Ives 29 for red (650 nm) and green (510 nm) flicker stimuli viewed foveally. The green light function is steeper by 12.4 percent. ( C ) CFF data for red (660 nm) and green (540 nm) flickering stimuli for a single observer tested by Pokorny & Smith. 30 The green light function is steeper by 16.8 percent. ( D ) CFF data for green light (mean of 480 and 540 nm data) and red light (630 nm) flicker from Giorgi. 27 The green light function is steeper than the red by 7.8 percent.

Effect of eccentricity on CFF

The CFF varies as a function of eccentricity in the visual field. If the stimulus size and luminance are kept constant, the CFF increases with eccentricity over the central 50 degrees or so of the visual field and then decreases with further increases in eccentricity. This is illustrated in Figure 37.5 , which plots the CFF as a function of eccentricity in the temporal visual field.

Figure 37.5, Critical flicker frequency (mean ± standard deviation) as a function of eccentricity in the temporal visual field of the right eye of one observer. (Reproduced with permission from Rovamo & Raninen 1984. 34 ) Stimulus area was 88.4 degrees 2 , pupil diameter was 8 mm, and retinal illuminance was 2510 photopic trolands.

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