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Tom C A Freeman

Publications and source records attributed to Tom C A Freeman.

8 recordsLinked to original sources
PubMed2005

Extra-retinal adaptation of cortical motion-processing areas during pursuit eye movements.

Repetitive eye movement produces a compelling motion aftereffect (MAE). One mechanism thought to contribute to the illusory movement is an extra-retinal motion signal generated after adaptation. However, extra-retinal signals are also generated during pursuit. They modulate activity within cortical motion-processing area MST, helping transform retinal motion into motion in the world during an eye movement. Given the evidence that MST plays a key role in generating MAE, it may also become indirectly adapted by prolonged pursuit. To differentiate between these two extra-retinal mechanisms we examined storage of the MAE across a period of darkness. In one condition observers were told to stare at a moving pattern, an instruction that induces a more reflexive type of eye movement. In another they were told to deliberately pursue it. We found equally long MAEs when testing immediately after adaptation but not when the test was delayed by 40 s. In the case of the reflexive eye movement the delay almost completely extinguished the MAE, whereas the illusory motion following pursuit remained intact. This suggests pursuit adapts cortical motion-processing areas whereas unintentional eye movement does not. A second experiment showed that cortical mechanisms cannot be the sole determinant of pursuit-induced MAE. Following oblique pursuit, we found MAE direction changes from oblique to vertical. Perceived MAE direction appears to be influenced by a subcortical mechanism as well, one based on the relative recovery rate of horizontal and vertical eye-movement processes recruited during oblique pursuit.

Adaptation, Ocular↗
PubMed2004

Perceiving depth order during pursuit eye movement.

Pursuit eye movements alter retinal motion cues to depth. For instance, the sinusoidal retinal velocity profile produced by a translating, corrugated surface resembles a sinusoidal shear during pursuit. One way to recover the correct spatial phase of the corrugation's profile (i.e. which part is near and which part is far) is to combine estimates of shear with extra-retinal estimates of translation. In support of this hypothesis, we found the corrugation's spatial phase appeared ambiguous when retinal shear was viewed without translation, but unambiguous when translated and viewed with or without a pursuit eye movement. The eyes lagged the sinusoidal translation by a small but persistent amount, raising the possibility that retinal slip could serve as the disambiguating cue in the eye-moving condition. A yoked control was therefore performed in which measured horizontal slip was fed back into a fixated shearing stimulus on a trial-by-trial basis. The results showed that the corrugation's phase was only seen unambiguously during the real eye movement. This supports the idea that extra-retinal estimates of eye velocity can help disambiguate ordinal depth structure within moving retinal images.

Cues↗
PubMed2003

The extra-retinal motion aftereffect.

Repetitive eye movements are known to produce motion aftereffect (MAE) when made to track a moving stimulus. Explanations typically centre on the retinal motion created in the peripheral visual field by the eye movement. This retinal motion is thought to induce perceived motion in the central test, either through the interaction between peripheral MAE and central target or by adaptation of mechanisms sensitive to the relative motion created between centre and surround. Less attention has been paid to possible extra-retinal contributions to MAE following eye movement. Prolonged eye movement leads to afternystagmus which must be suppressed in order to fixate the stationary test. Chaudhuri (1991, Vision Research, 131, 1639-1645) proposed that nystagmus-suppression gives rise to an extra-retinal motion signal that is incorrectly interpreted as movement of the target. Chaudhuri's demonstration of extra-retinal MAE depended on repeated pursuit to induce the aftereffect. Here we describe conditions for an extra-retinal MAE that follows more reflexive, nystagmus-like eye movement. The MAE is extra-retinal in origin because it occurs in part of the visual field that received no retinal motion stimulation during adaptation. In an explicit test of the nystagmus-suppression hypothesis, we find extra-retinal MAE fails to store over a 30s delay between adaptation and test. Implications for our understanding of motion aftereffects are discussed.

Eye Movements↗
PubMed2003

Optokinetic potential and the perception of head-centred speed.

Extra-retinal information about eye velocity is thought to play an important role in compensating the retinal motion experienced during an eye movement. Evidently this compensation process is prone to error, since stimulus properties such as contrast and spatial frequency have marked effect on perceived motion with respect to the head. Here we investigate the suggestion, that 'optokinetic potential' [Perception 14 (1985) 631] may contribute to an explanation of these errors. First, we measured the optokinetic nystagmus induced by each stimulus so as to determine the optokinetic potential. Second, we determined the speed match between two patches of Gaussian blobs presented sequentially. Observers pursued the first pattern and kept their eyes stationary when viewing the second. For stimuli with identical contrast or spatial frequency, the pursued pattern was perceived to move slower than the non-pursued pattern (the Aubert-Fleischl phenomenon). Lowering the contrast or the spatial frequency of the non-pursued pattern resulted in a systematic decrease of its perceived speed. A further condition in which the contrast or spatial frequency of the pursued pattern was varied, resulted in no change to its perceived speed. Pursuit eye movements were recorded and found to be independent of stimulus properties. The results cast doubt on the idea that changing contrast or spatial frequency affects perceived head-centred speed by altering optokinetic potential.

Eye Movements↗
PubMed2003

Horizontal rhythmical eye movements consistently diminish the arousal provoked by auditory stimuli.

OBJECTIVES: Theoretical models implicating the orienting reflex as an explanatory mechanism in the eye-movement desensitization and reprocessing (EMDR) treatment protocol are contrasted and tested empirically. We also test whether EMDR effects are due to a distraction effect. DESIGN: A repeated measure design is used in two experiments. The first experiment employed two independent variables, eye condition (moving vs. stationary) and tone (a pseudo-randomized series of low and high intensity tones). In Expt 2, eye condition was replaced by attentional demand conditions (low or high). In both cases, electrodermal responses served as the dependent variable. METHOD: Participants were recruited from the Psychology Department at Cardiff University. In Expt 1, participants were required to either pursue a moving stimulus following auditory challenge or engage in an eyes-stationary task. In Expt 2, the task following auditory challenge required participants to identify specific items from letter strings in low and high attentional demand conditions. RESULTS: Lower levels of electrodermal arousal were identified in tasks eliciting eye movements, compared to no eye movements. This effect was not due to the attentional requirements of the task. CONCLUSIONS: Eye movements following auditory challenge result in an effect of psychophysiological de-arousal. This supports the reassurance reflex model of EMDR proposed by MacCulloch and Feldman (1996).

Acoustic Stimulation↗
PubMed2002

Motion versus position in the perception of head-centred movement.

Abstract. Observers can recover motion with respect to the head during an eye movement by comparing signals encoding retinal motion and the velocity of pursuit. Evidently there is a mismatch between these signals because perceived head-centred motion is not always veridical. One example is the Filehne illusion, in which a stationary object appears to move in the opposite direction to pursuit. Like the motion aftereffect, the phenomenal experience of the Filehne illusion is one in which the stimulus moves but does not seem to go anywhere. This raises problems when measuring the illusion by motion nulling because the more traditional technique confounds perceived motion with changes in perceived position. We devised a new nulling technique using global-motion stimuli that degraded familiar position cues but preserved cues to motion. Stimuli consisted of random-dot patterns comprising signal and noise dots that moved at the same retinal 'base' speed. Noise moved in random directions. In an eye-stationary speed-matching experiment we found noise slowed perceived retinal speed as 'coherence strength' (ie percentage of signal) was reduced. The effect occurred over the two-octave range of base speeds studied and well above direction threshold. When the same stimuli were combined with pursuit, observers were able to null the Filehne illusion by adjusting coherence. A power law relating coherence to retinal base speed fit the data well with a negative exponent. Eye-movement recordings showed that pursuit was quite accurate. We then tested the hypothesis that the stimuli found at the null-points appeared to move at the same retinal speed. Two observers supported the hypothesis, a third partially, and a fourth showed a small linear trend. In addition, the retinal speed found by the traditional Filehne technique was similar to the matches obtained with the global-motion stimuli. The results provide support for the idea that speed is the critical cue in head-centred motion perception.

Eye Movements↗
PubMed2002

Head-centred motion perception in the ageing visual system.

Stationary objects appear to move in the opposite direction to a pursuit eye movement (Filehne illusion) and moving objects appear slower when pursued (Aubert-Fleischl phenomenon). Both illusions imply that extra-retinal, eye-velocity signals lead to lower estimates of speed than corresponding retinal motion signals. Intriguingly, the velocity (i.e. speed and direction) of the Filehne illusion depends on the age of the observer, especially for brief display durations (Wertheim and Bekkering, 1992). This suggests relative signal size changes as the visual system matures. To test the signal-size hypothesis, we compared the Filehne illusion and Aubert-Fleischl phenomenon in young and old observers using short and long display durations. The trends in the Filehne data were similar to those reported by Wertheim and Bekkering. However, we found no evidence for an effect of age or duration in the Aubert-Fleischl phenomenon. The differences between the two illusions could not be reconciled on the basis of actual eye movements made. The findings suggest a more complicated explanation of the combined influence of age and duration on head-centred motion perception than that described by the signal-size hypothesis.

Adult↗