W13: Temporal Vision

Temporal Vision

Learning Objectives

• Understand the change in visual sensitivity to variations in light information over time (temporal sensitivity).

• Measurement of temporal sensitivity with flickering lights.

• Temporal sensitivity and its relationship to the Impulse Response (IRF).

• Characteristics of human temporal sensitivity (including chromatic, photopic, mesopic & scotopic).

• Physiological basis of temporal contrast sensitivity.

• Factors affecting temporal contrast sensitivity in humans.

• Effect of disease on temporal vision.

Measurement of Temporal Sensitivity with Flickering Lights

Temporal Contrast Sensitivity (TCS)

• Measure of sensitivity to variations in luminance and/or chromaticity over time (also known as flicker sensitivity).

• Spatial contrast sensitivity measures sensitivity to variations in luminance and/or chromaticity over space.

• TCS functions can be measured using any stimulus that changes over time (e.g., sinusoidal or rectangular waveforms).

Sinusoidal Flicker

• Quantified using parameters similar to those for a sinusoidal grating pattern:

  • Amplitude (A) or Contrast (C) of the modulation.

  • Mean Luminance (L_o or L_ave).

  • Temporal period (${2π/ω}$): Length of one full cycle

  • Unit: Frequency (Hz) in cycles per second.

• Temporal Modulation Depth: relationship between the peak and time average luminance.

• Michelson contrast:

  

• Threshold Amplitude = %Contrast x $L_{ave}$

Temporal Frequency

• Inverse of temporal period.

• Number of cycles per second (Hz).

Phase

• Relates the position of a feature (e.g., peak or trough) to that same feature in another part of the waveform.

Temporal Sensitivity and its Relationship to the Impulse Response Function (IRF)

• common for electroretinogram - gives an indication of impulse response, allowing the study of photoreceptor function.

Temporal Contrast Sensitivity Function

• Defined as the ability of the visual system to detect variations in the rate of change in light over time.

• Temporal contrast sensitivity (TCS): $TCS = 1 / Contrast Threshold$ @ each frequency (Hz).

• Temporal Contrast Sensitivity Function (TCSF)

  • low frequency = low contrast sensitivity

  • as frequency increases = contrast sensitivity increases

  • peak = human eye is sensitive to temporal changes that are less than 1% contrast.

• Critical Fusion Frequency (CFF): The frequency at which flicker is no longer perceived. ~100% modulation (frequency of 100)

3D spatio-TCSF:

• eye is most sensitive to temporal frequency ~4-8 Hz.

Measurement of Temporal Modulation Transfer Functions

• Temporal modulation transfer function introduced to human visual psychophysics from engineering.

• Measures transfer function (input/output amplitude & phase response) of an electronic system as a function of the temporal frequency of the sinusoidal input.

• Linear systems can be completely described by the impulse response.

  • non-linear system will have changes in phase or the frequency composition of the stimulus.

• In vision, assesses contrast modulation threshold as a function of temporal frequency of a sinusoidal waveform, assuming a constant criterion independent of temporal frequency.

• This format opens itself to modelling using linear systems (including Fourier analysis).

Fourier Theory

• Assumes a linear system:

  • The visual system behaves linearly at low stimulus contrasts (near threshold).

  • Sensitivity determined by the fundamental frequency in the stimulus (Eisner, 1995).

  

• Visual response to complicated temporal signals is the sum of responses to the individual temporal sinusoidal components (amplitude & phase) of the flicker pattern.

  • Any stimulus can be used to measure temporal sensitivity.

• when stimulus is rectangular, there is higher frequency energy contained at the edges. This produces energy at multiples of the harmonics.

• The shape of the waveform can impact the composition of the stimulus.

  
  

Impulse Response Functions (IRF)

• A delta function is a (theoretical) stimulus light with an infinitesimal short time & infinite energy; the Fourier spectrum of a delta function contains equal energy at all temporal frequencies (equally flat at all frequency)

  • can’t do it directly in humans due to physiological limits.

• An IRF completely defines the temporal response of a system (i.e., the output) to an (input) delta function.

• IRFs are specified in terms of amplitude & time-to-peak and are typically monophasic (up & down) or biphasic.

• lag caused by feedback from horizontal cells.

• For vision, we can only approximate the delta function with light stimuli, but can reliably measure visual TCSFs with different temporal frequency sinewaves.

• Importantly, the IRF of the human visual system can then be mathematically derived from the TCSF.

• When the IRF is known, visual sensitivity can be predicted for any stimulus.

Characteristics of Human Temporal Sensitivity

Photopic TCSF

• Light level has a significant impact on the form of the photopic TCSF.

  • Band-pass at high light levels.

  • Low-pass at low light levels.

    • 1 troland is one cone threshold on average (anything below is stimulating rods)

  • TCS worst at low and high Hz.

  • Peak sensitivity at 8–20 Hz.

  • TCT at peak can be <1%.

  • Temporal contrast sensitivity has high sensitivity because it activates both on and off pathways over time.

• Critical Fusion Frequency (CFF) is the max temporal frequency resolved by the visual system (assumes max modulation of the target):

  • High frequency cut-off up to 60 Hz.

  • as light level increases → CFF increases

Scotopic TCSF

• Light level alters the scotopic TCSF shape.

  • Band-pass at high light levels.

  • Low-pass at low light levels.

  • TCS worst at low and high Hz.

  • Peak sensitivity at ~ 4–9 Hz.

  • CFF for rod responses as high as 28 Hz.

Temporal Contrast Sensitivity and the Impulse Response Function (IRF)

• With decreasing light level:

  • IRF changes from biphasic to monophasic.

  • Time-to-peak is delayed.

• biphasic = band pass function

  • the faster the impulse response → the more band pass → higher the peak of temporal contrast sensitivity → the higher the CSF

• monophasic = low band pass.

  • as light level goes down, CS shifts towards lower frequencies and it takes longer to reach the peak.

    • amplitude of impulse response is slower / smaller.

Luminance and Chromatic TCS

• Bandpass TCSF for luminance.

• Lowpass TCSF for chromatic.

• Biphasic IRF = bandpass TCSF.

• Monophasic IRF = lowpass TCSF.

Photoreceptor-isolating spatiotemporal vision

• L+M: magnocellular pathway

  • achromatic system has high temporal contrast sensitivity and good spatial sensitivity, but it is a band pass function

• L-M: parvocellular pathway

  • chromatic red/green very good at low temporal frequency.

• S/L+M: bistratified ganglion cells for koniocellular pathway

  • sensitivity is reduced.

• rods: spatiotemporal contrast modulations at photopic light levels, however, are much less sensitive than all of the different post receptor pathways.

• melanopsin: excellent contrast sensitivity in regions where the cones are less sensitive.

• As stimuli change, the visual system shifts between achromatic processes, red, green, blue and yellow chromatic process, rods and melanopsin - vision remains clear throughout the environment.

  • no sudden shock = smooth transition.

Representing TCSF Data

contrast or amplitude?

• TCS (or threshold) is plotted on the y-axis & temporal frequency on the x-axis using log–log axes.

• Threshold Amplitude vs. Temporal frequency (log–log plot), where threshold amplitude is the product of temporal contrast and mean luminance of the target.

Characteristics of TCS: Linearity & Weberian Behaviour

• High temporal frequency performance

  • Linear relationship between frequency & threshold amplitude

  • Ferry–Porter law applies (CFF ∝ Log Td)

    • retinal illuminance in trolands = Td

• Low temporal frequency performance

  • TCSF curves converge at different retinal illuminances when plotted on contrast

  • Weberian Behaviour (ΔL/L=k)

    • k = constant

Talbot-Plateau Law

• For flicker beyond the CFF: When Hz > CFF, appearance (brightness) equal to steady light of same $L_{ave}$.

• example: TV and displays.

Ferry-Porter Law

• Relates how CFF varies with retinal illuminance

  • CFF proportional to log retinal illuminance, expressed as:

    • $f = alogE + b$

    • f = CFF (Hz)

    • E = retinal illuminance (Trolands)

    • a and b are constants

• Rod and Cone portions

  • Rods: maximum CFF ~ 28 Hz

    • initial low frequency component ~16Hz, followed by linear increase of 28Hz.

    • explained by lower limb being related to the high sensitive rod bipolar pathway. The higher frequency limb is related to the transmission of the rod signals through the cone pathway.

  • Cones: maximum CFF ~ 80 Hz

• L+M cone (luminance) & S-cone CFF’s follow the Ferry-Porter law

• Melanopsin-mediated CFF is invariant with light level - doesn’t change its temporal frequency.

Granit-Harper Law

• Relates how CFF varies with target area

  • CFF proportional to logarithm of target size, expressed as:

    • $F = clogA + d$

    • F = CFF (Hz)

    • A = target area ($m^2$)

    • c and d are constants

  • larger the stimulus area = higher the CFF

Physiological Basis of Human and Primate Temporal Contrast Sensitivity

Electrophysiological evidence: Macaque (monkey) Photoreceptor Impulse Response Functions (IRF)

• Photoreceptor (& post-receptoral pathway) responses are intensity dependent

  • As stimulus intensity increases, the latency (i.e., the time taken to react) of the response decreases as does its duration

  • Thus, a quicker responding photoreceptor will allow a higher CFF

• Similar changes in photoreceptor IRF and behavioural IRF with changes in intensity (i.e., becomes faster and larger with increasing intensity)

  • TCSF can be explained by the IRF

tCSF Model: Smith, Pokorny, Lee & Dacey (2008)

• A quasi-linear* systems approach can be used to describe the interaction of temporal dynamics and sensitivity regulation (adaptation) as a function of illumination

  • *quasi-linear: relationship between input-output signal is substantially linear despite existence of nonlinear elements

• Primate tCS data measured in H1 Horizontal cells (reflect photoreceptor output), MC and PC cells.

• Human tCS data measured for chromatic and luminance modulations

Macaque monkey

• H1 model incorporated as an early time-dependent stage of sensitivity regulation

• The ganglion cell model incorporates center–surround subtraction (in MC & PC)

• MC-pathway shows an additional time-dependent stage of sensitivity regulation that results in Weber’s Law

• Chromatic PC cells form a band pass function.

• MC cells has a high temporal frequency sensitivity. Also is band-pass. Yellow bar → Weberian Behaviour is present in MC cells (this is not seen in PC cells).

Human data:

• Sensitivity regulation preserved in human thresholds, but sensitivity reduced at high Hz: implies cortical mechanisms limit thresholds at high Hz, especially in the PC pathway

• Sub-Weberian behaviour in chromatic modulations

• Weberian behaviour in achromatic at low frequency + high sensitivities to high frequencies.

• Brain is masking out the low frequency filter →Temporal frequency signals at high frequencies that are present for chromatic stimuli.

Factors Affecting Temporal Contrast Sensitivity

Retinal Location

• CFF poorer in periphery for small targets (1–2°diameter)

• High retinal illuminance: CFF best at fovea

• Low retinal illuminance: CFF best in periphery

  • for photopic levels.

  • rods are more sensitive in periphery.

Illumination, Wavelength and CFF

• Photopic

  • CFF independent of wavelength with retinal illuminance (normalized to $V_λ$)

• Scotopic

  • CFF varies with wavelength if photopic $V_λ$ is used instead of scotopic $V^{\prime}_{\lambda}$

• Mesopic

  • Rod-cone interactions alter temporal visual sensitivity

  • destructive interferences = reduces temporal sensitivity.

Photoreceptor interaction: Mesopic Lateral Rod Suppression of Cone CFF

• Difference between Light Adapted (LA) and Dark Adapted critical fusion frequency (CFF) indicate rod-cone interactions are strongest for L-cone, M-cone and luminance-containing modulations (LMS, LMSR), consistent with strong interactions in the MC-pathway.

Lateral Rod Suppression of Cone Temporal IRFs

• Rod activity reduces cone pathway IRF amplitude & delays the time to peak

• IRF changes associated with reduced MC-pathway contrast sensitivity & longer visual processing times; has direct implications for night-time driving (reaction times)

Affect of Disease on Temporal Contrast Sensitivity

Mesopic CFF in persons with high risk genotypes for ARM

• Retinal metabolic demand is higher in dim lighting; flickering lights increase processing requirements, therein the CFF is sensitive to early disease changes

• People with normal visual acuity, no ophthalmoscopically signs of ARM, and who are carriers of the CFH, LOC387715, & HRTA1 high-risk genotypes (gene-positive) have impaired rod- and cone-mediated mesopic visual function compared with persons who do not carry the risk genotypes (gene-negative)

Dichromacy

• Dichromatic retina: Deuteranope CFF > Protanopic CFF

• Difference in CFF increases with increasing light level

Temporal Sensitivity in Glaucoma & Optic Neuritis

• Glaucoma: Generalized (non-selective) loss at all frequencies (central & peripheral)

• Optic neuritis: Non-selective spatial/temporal frequency loss, although individual patients can show selective deficits

Temporal Sensitivity in ARM and Diabetes

• Flicker stimuli are more sensitive for detecting sensitivity losses than static (pulsed) stimuli – important for early detection

• ARM: perimetry normal in peripheral but decreases in central retina (for static). Loss of in central (7-8 deg) in flicker

• Diabetes: Generalised loss across all retinal areas.

• Difference: flicker produces a larger loss compared to the static. This means there can be an advantage for using a temporally modulating target to detect early changes.

Temporal Sensitivity in Migraineurs

• Deficit is observed with flickering stimuli (but not static stimuli)

• Visual field in all quadrants with flicker.

• Migraineurs can have a reduced temporal sensitivity for over a month following migraines.

Questions to ask yourself: Key concepts

• How does the shape of the TCSF change with illumination level?

• Through what assumptions can the TCSF and IRF be linked?

• What are the major physiological differences between the rod & cone photoreceptor IRFs?

• How does the change in CFF with retinal location and illuminance depend on the rod and cone photoreceptors?

• The human CFF can be described by what three laws?

• Consider the clinical applications of flicker stimuli in the early detection of disease