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T Maddess

Publications and source records attributed to T Maddess.

At least 19 recordsLinked to original sources
PubMed2001

Discriminating isotrigon textures [corrected].

Higher order spatial correlations can capture edge and object relationships. Isotrigon textures are useful for studying our sensitivity to these correlations. We determined human discrimination performance for 18 isotrigon texture types and compared it with outputs from statistical discriminant models. Some of the models employed versions of the Allan Variance in receptive field outputs. Physiologically plausible mechanisms for such calculations are presented. Two discriminant models emulated human performance well, one based upon a global variance measure, and the other based upon a localised variance with an orientation bias. The 18 texture types were also shown to contain characteristic mini-textures.

Adult↗
PubMed2001

Comparison of three tests using the frequency doubling illusion to diagnose glaucoma.

PURPOSE: The introduction of the FDT perimeter prompted the comparison of three tests employing frequency doubling (FD) stimuli. These measures compared different visual field locations and contrast ranges. Frequency of seeing curves were examined for the method most similar to FDT. METHODS: For 146 eyes the following were obtained: (i) contrast matches to two suprathreshold FD stimuli (normal subjects, ocular hypertensve suspects, primary open angle glaucoma subjects); (ii) two alternative forced choice (2AFC) thresholds for horizontally versus vertically orientated FD gratings: and (iii) contrast thresholds determined by method of adjustment (MOA) for five different stimulus types. RESULTS: A model based on the worst of the MOA hemifield thresholds performed best. The suprathreshold contrast matching tests performed worst. Frequency of seeing curves were fitted for the 146 eyes of the 2AFC tests. Although the MOA thresholds were higher than the 2AFC thresholds (for normals mean +/- SE, 8.47 +/- 0.43 dL, P < 0.0000), the best diagnostic concordance was at lower limens (75% or 80% correct) of the fitted frequency of seeing curves. CONCLUSIONS: There was good diagnostic concordance between the MOA and 2AFC methods although the thresholds were 1.8-fold different on a log-scale. This suggests that the same neural mechanism mediates both thresholds for rapidly flickering, spatially coarse, patterns.

Adult↗
PubMed2000

Spectral sensitivity of photoreceptors in an Australian marsupial, the tammar wallaby (Macropus eugenii).

Microspectrophotometric measurements on the rod photoreceptors of the tammar wallaby showed that they have a peak absorbance at 501 nm. This indicates that macropod marsupials have a typical mammalian rhodopsin. An electroretinogram-based study of the photoreceptors confirmed this measurement and provided clear evidence for a single middle wavelength-sensitive cone pigment with a peak sensitivity at 539 nm. The electroretinogram did not reveal the presence of a short-wavelength-sensitive cone pigment as was expected from behavioural and anatomical data. Limitations of the electroretinogram in demonstrating the presence of photopigments are discussed in relation to similarly inconsistent results from other species.

Animals↗
PubMed2000

Employing following eye movements to discriminate normal from glaucoma subjects.

We recorded optokinetic nystagmus (OKN) to see if slow phase velocity, duration or other measures were affected by glaucoma. Drifting grating patterns that either weakly or strongly evoked the spatial frequency doubling illusion were employed. Analysis of 68 variables characterizing the OKN revealed that small subsets of these variables were good at discriminating normal from primary open angle glaucoma subjects. The variables were related to the regularity of following eye movements. Models including the best five variables selected in two different ways classified about 90% of subjects correctly. Impaired accuracy of eye movements suggests that glaucoma changes the signal to noise ratio available to the brain. The gross changes observed permit the use of electro-oculography or other simple methods in the clinic.

Diagnostic Techniques, Ophthalmological↗
PubMed2000

A spatial frequency-doubling illusion-based pattern electroretinogram for glaucoma.

PURPOSE: A pattern electroretinogram (PERG) in which stimuli displaying the frequency-doubling (FD) illusion are presented simultaneously to multiple parts of the visual field was evaluated for its ability to diagnose glaucoma. This multiregion FD PERG is referred to in the current study as the MFP. METHODS: The nine stimulus regions were temporally modulated at incommensurate frequencies typically producing an FD percept. Two other spatial scales of the stimuli were also investigated. The sensitivity and specificity of MFP were examined using linear and quadratic discriminant methods. RESULTS: Even with the simpler linear discriminant classification, sensitivities and specificities of 100% were obtained in eyes with moderate to severe glaucoma. Of eyes with glaucoma strongly suspected, 67% were classified as being glaucomatous. Stimulus patterns having differing spatial scales produced different PERG visual field dependencies. CONCLUSIONS: The differing results for the 16-fold change in spatial scale may reflect the accessing of different mechanisms. The MFP method appears to have significant value for the diagnosis of glaucoma.

Electroretinography↗
PubMed2000

Comparing a parallel PERG, automated perimetry, and frequency-doubling thresholds.

PURPOSE: A pattern electroretinogram (PERG) simultaneously displaying the frequency-doubling (FD) illusion in nine parts of the visual field was compared with two other methods for ability to detect glaucoma. This multiregion FD PERG (MFP) was compared with results from achromatic automated perimetry and psychophysical tests using FD stimuli. METHODS: MFP data were compared with that from the Humphrey Field Analyser (HFA; Humphrey, San Leandro, CA) 24-2 program. Contrast thresholds were also determined in different visual field locations for FD stimuli. Thin-plate spline methods were used to derive comparisons from the tests, each of which sampled the visual field differently. RESULTS: Significant correlation with HFA could be obtained, providing seven to nine (of nine) MFP amplitudes were themselves significant. Evidence showed that both the psychophysical tests using FD stimuli and the MFP detect glaucomatous damage not detected by the HFA. CONCLUSIONS: The comparisons between HFA perimetry, the MFP, and FD thresholds indicate that both FD-based tests quantify a form of diffuse loss in early glaucoma as well as the scotomas of later glaucoma.

Electroretinography↗
PubMed1999

Apparent fineness of stationary compound gratings.

Patterns consisting of the sum of a sinusoidal grating and its second spatial harmonic have an apparent spatial fineness, or periodicity, that is about halfway between the two component spatial frequencies. There are also phase dependent modulations of the apparent fineness about the mean fineness shift. Covariance between individuals' phase dependent fineness shifts indicates the presence of four spatial phase channels. The apparent fineness effects, and the putative phase channels, may both be a product of a local, linear, analysis of spatial frequency content. Illusory second harmonics, as generated in the spatial frequency doubling illusion, also change apparent fineness.

Contrast Sensitivity↗
PubMed1999

Testing for glaucoma with the spatial frequency doubling illusion.

We examined the performance of tests for glaucoma based on the spatial frequency doubling (FD) illusion. Contrast thresholds for seeing the FD illusion in four large visual field regions were measured from 340 subjects who were tested up to seven times over 2 years. Median sensitivities of 91% at specificities of 95% were obtained. Test-retest variability for the worst hemifield thresholds averaged 2.22 db +/- 0.09 S.E. for all tested groups, and significant progression was observed for glaucoma suspects over the seven visits, indicating that tests based on the FD illusion can detect diffuse early glaucomatous loss.

Adult↗
PubMed1998

The spatiotemporal properties of the Craik-O'Brien-Cornsweet effect are consistent with 'filling-in'.

The Craik-O'Brien-Cornsweet effect (COCE) is an illusion in which luminance discontinuities give rise to illusory brightness. One hypothesised mechanism for the induction of illusory brightness is that the cortex constructs a brightness percept from edge information by a lateral 'filling-in' process. A requirement for the filling-in hypothesis is that ability of the illusion to form would be limited by the speed of propagation of the filling-in. The results presented here from three methods indicate that in the case of COCE gratings brightness information propagates at a fixed speed across the central visual field of about 19 degrees/s, and across visual areas V1 or V2 at 155 or 205 (+/- 20) mm/s, respectively.

Contrast Sensitivity↗
PubMed1998

Evidence for spatial aliasing effects in the Y-like cells of the magnocellular visual pathway.

Several lines of evidence are provided indicating that our visual percept can be dominated by spatial aliasing for viewing conditions near those needed to see the spatial frequency doubled illusion. The apparent aliasing effect indicates that the underlying sampling array has a density 15-30% of that of M-cells, in agreement with the known proportion of Y-like M-cells (M(y)-cells). The presence of aliasing indicates, that there is a separate irregular array of M(y)-cells, and that their role is to rapidly convey information on retinal gain control to the brain rather than to act primarily as inputs to image motion computation.

Adaptation, Ocular↗
PubMed1998

The Craik-O'Brien-Cornsweet effect and brightness induction both proceed by the spreading of brightness information.

BACKGROUND: The Craik-O'Brien Cornsweet effect (COCE) is a visual illusion where the luminance of image boundaries sets the apparent brightness of enclosed regions. The COCE may be produced by the cortex constructing the observed brightness through a lateral 'filling-in' process: propagating brightness information from the edges of the enclosed regions towards their centres. Any such filling-in process would imply a speed of propagation. METHODS: Data on the propagation speed of brightness information in two different brightness induction effects are compared using a multivariate regression analysis. RESULTS/CONCLUSION: We demonstrate similar non-zero speeds for the COCE and for a brightness contrast effect.

Contrast Sensitivity↗
PubMed1997

Correlations between observability of the spatial frequency doubled illusion and a multi-region pattern electroretinogram.

A glaucoma screening device based on the visibility of the spatial frequency doubled (FD) illusion will be marketed by Welch Allyn Ltd in the next year (ANU Patients (Australia) 611,585, (USA) 5,065,767 and application PL 3130). An underlying assumption of the method is that retinal processes are being tested. To test this assumption we compared the visibility of the FD illusion over a range of conditions and in the same spatial locations as a multi-region pattern electroretinogram (PERG). Grating speed and contrast were good predictors of the psychometric functions and PERG amplitude and phase.

Electroretinography↗
PubMed1997

A multiple-frequency, multiple-region pattern electroretinogram investigation of non-linear retinal signals.

It has been proposed that the spatial frequency doubled (FD) illusion may originate from Y-like non-linear retinal ganglion cells. If the contrast of multi-frequency stimuli is increased, Y cells show a phase advance in the self-sum frequencies but not in other output frequencies. We looked for these effects with a multi-region pattern electroretinogram (PERG) displaying the sum of two temporal frequencies in each visual field location. Regional variation was found in the recorded sum and difference frequencies. The results indicate that PERG signals become dominated by responses from Y-like cells when the FD illusion is seen.

Contrast Sensitivity↗
PubMed1994

The effects of adaptation to visual stimuli on the velocity of subsequent ocular following responses.

We examined the effect of prior adaptation to moving and flickering stimuli on the velocity of subsequent ocular following responses in man. Experiments consisted of two phases: an adaptation phase in which moving or flickering stimuli were presented while the eyes fixated a small spot and a test phase in which ocular following responses were free to occur. The effects resulting from prior adaptation were characterized by determining the mean initial eye velocities in the period 200-500 ms after the onset of the test stimulus. It was found that 8 s of prior exposure to a grating pattern moving at between 1.5 and 4 cycles.s-1 significantly reduced initial eye velocities in all subjects. Prior exposure to a flickering stimulus (temporal frequency 3.2 cycles.s-1) also attenuated the velocities of initial eye movements, but to a far lesser extent. These results suggest that a motion-dependent and a weaker flicker-dependent process have an adaptive influence on the generation of ocular following responses. Initial eye velocities were measured as a function of the contrast of the prior adapting gratings. The velocities were found to decrease with increasing adapting contrast. The reductions in eye velocity were well described by a decaying exponential function. The motion-dependent adaptive effect showed significant inter-ocular transfer and had the same temporal tuning when transferred (i.e. optimum adaptation at between 1.5 and 4 cycles.s-1). The flicker-dependent effect did not show inter-ocular transfer. There is a distinct similarity between the adaptive process that causes attenuation of ocular following velocities and the adaptive mechanism that induces perceptual motion after-effects. This similarity is discussed.

Acceleration↗
PubMed1992

Human ocular following responses are plastic: evidence for control by temporal frequency-dependent cortical adaptation.

Optokinetic nystagmus (OKN) induced by wide-field visual stimulation was measured with and without prior adaptation to moving sinusoidal gratings. Under unadapted conditions the mean gains of the slow phases of OKN in the first 500 ms were 0.5-0.8, and the eye velocities and amplitudes had rise times with time constants of 0.1-0.2 s. By contrast, following adaptation to as little as 1 s of image motion, the magnitude of the initial gains fell and the rise times of the velocities and amplitudes increased markedly. The degree of adaptation depended on the adapting temporal frequency, the optimum adaptive frequencies being 1.7-3.4 Hz. In this range of temporal frequencies, the initial gains fell to 0.1-0.3 and the rise times for velocity and amplitude ranged from 0.4 to 7.0 s, depending on the length of the adapting period. Thus the observed changes in the time constant were up to 70-fold. Neither spatial frequency or image velocity had any marked influence on the level of adaptation. The dependence on temporal frequency rather than image velocity suggests that the motion detectors feeding the adaptive system respond to local motion-related changes in luminance. The adaptive effects were direction-selective, showing that this must also be the case for the motion detectors. The adaptive effects were observed both when the drift temporal frequency on the retina was established by artificially maintaining a fixed gaze or when the adapting temporal frequency was induced by retinal slip during OKN. Time constants for recovery from adaptation were similar to motion aftereffects measured by psychophysical and physiological methods. The results suggest a link between cortical motion adaptation and adaptive mechanisms effecting the oculomotor system.

Adaptation, Physiological↗
PubMed1988

Factors governing the adaptation of cells in area-17 of the cat visual cortex.

Neurons in area 17 of the cat visual cortex adapt when stimulated by drifting patterns of optimal orientation, spatial frequency and temporal frequency (Ohzawa et al. 1982; Albrecht et al. 1984; Ohzawa et al. 1985). A component of this adaptation has been attributed to a contrast gain-control mechanism, rather than to neural fatigue, and results in enhanced differential sensitivity around the adapting contrast level (Ohzawa et al. 1982; Albrecht et al. 1984; Ohzawa et al. 1985). Experiments described here suggest that neural response rate, the directional selectivity of the cell, and the temporal frequency of the stimulus, are the principal determinants of adaptation, irrespective of other stimulus parameters such as contrast, velocity, or spatial frequency. The present results can nevertheless accommodate the results of previous studies of adaptation, and additionally provide scope for the resolution of apparent contradictions between results from psychophysical and neurophysiological studies of adaptation.

Adaptation, Physiological↗
PubMed1986

Afterimage-like effects in the motion-sensitive neuron H1.

A powerful effect resembling an afterimage is demonstrated on the pathway to the motion-sensitive neuron H1. This effect is independent of the locally generated gain control described in an earlier paper (Maddess & Laughlin 1985, Proc. R. Soc. Lond. B 225, 251). The afterimage, produced across the eye by a stationary pattern, causes the sensitivity to movement to be different according to the local stimulus history, and the effects of low-contrast (0.1) patterns, presented for as little as a few hundred milliseconds, remain for up to 2 s. Moving patterns interact with the afterimage to modulate the spike rate of H1. The afterimage increases with contrast but saturates at contrasts above 0.5. Low spatial frequencies generate afterimages less effectively than moderate ones; this result indicates that the afterimage process could lie at, or after, lateral inhibition between tonic units. This is supported by the fact that the altered sensitivity profiles generated by single bright and dark vertical bars initially resemble Mach bands. However, this character alters as the afterimage decays, and the depression of H1's response to moving bright stimuli, produced by the afterimage of a dark bar, continues to grow for up to 1 s after the adapting bar is removed. A short-lived (0.5 s) reduction of H1's directional selectivity accompanies strong afterimage formation. All these factors, especially the saturation at low contrasts and the spatial frequency tuning, rule out light adaptation by photoreceptors as the afterimage source. Luminances used were also low enough to exclude influence by the pupil mechanism. Lastly, responses to patterns that are occasionally jumped by large or small distances are broadened by stimuli that produce an afterimage. Responses to small displacements have previously been described as 'velocity impulse responses' (Srinivasan 1983, Vision Res. 23, 659; Zaagman et al. 1983, IEEE Trans. SMC 13, 900) and so the response broadening (stimulus blurring) can be taken as a reduction of the fly's temporal resolution of moving objects. Previously reported work shows that afterimages seen in humans and the effect reported here act over the same range of temporal frequencies rather than retinal drift speeds. This may suggest an important role for afterimage-like effects in the processing of the low temporal frequency components of moving images. Certainly, the fly's afterimage system reduces the visibility of moving objects within patches of an image that, have on average, contained slowly varying motion signals.(ABSTRACT TRUNCATED AT 400 WORDS)

Afterimage↗