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Perception - Adam Merrin

Neural Processing Limits (18/2/13)

Adam Merrin

on 21 February 2013

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Transcript of Perception - Adam Merrin

Perception Adam Merrin Papers Synchronous Information Presented in 40-HZ Flicker Enhances Visual Feature Binding
Elliot & Müller (1998)

Spatial Structure Affects Temporal Judgements: Evidence for a Synchrony Binding Code
Cheadle & Bauer et al (2008) Binding Problem and Binding by Synchrony Hypothesis In 1986, Kittens had their visual cortex analysed in response to objects, with the finding that spatially separate areas of the brain fired in sync.
Coded by the Cerebral Cortex, attention based discrimination tasks gave evidence towards the BBS hypothesis.
Yet oscillating patterning of neural responses and the synch of these is stimulus dependent and highly sensitive to context. CONTROVERSY - Whether oscillating patterns actually exist and whether precise sync of discharges a coincidence or genuinely relevant to the debate. Elliott and Müller Method They aimed to provide the first psychophysical support for the theory of preattentive coding using a feature binding mechanism.
Using a Kanizsa-type figure paradigm, present information at varying degrees of hertz and presentation times. Internal Synchronizing Mechanisms and Hertz Experiment Conditions Experiment 1 - Reaction Time to the presence of a Kanizsa square in the matrix
Experiment 2 - Viewing potential phenomenal awareness
Experiment 3 - Similar to Experiment 1 yet at fixed frequency 1) Their is speculation over attentive states influencing detection and coding of a target, and that stimulus like texture, boundaries and other stimulus characteristics compete with temporal stimulus characteristic. Elliott and Müller - Methodological Considerations To counter this, a Premask paradigm (3x3 oscillating matrix).
The frequency could be manipulated whle creating a relationship between temporally coded premask information and figural information within the target.
The elements were seemingly simultanious yet presented asynchronously to other elements. Synchronous Random (Pseudorandom Arrangement) 3) To avoid participants becoming aware of the synchronous prestructure within the paradigm, a pilot was used to decipher the minimal frequency. Evidence has shown that at certain frequencies, the brain conducts different processes.
In visuo-cortical and sub-cortical structure, after the onset of a stimulus, the brain produces activity in the upper beta and gamma frequencies (20-70hz) When information is presented at these frequencies, it resembles our internal synchronizing mechanism, and therefore is coded better through the process of our binding mechanism. 2) Due to evidence on ocsillation ranges optimum for psychophysical operations, the suspected range of 30-60hz was redefined to 30-50hz. The premask paradigm was to be tested at 25, 29, 33, 40, 50, 67 and 100hz (premask frames per second). Pilot Varying premask paradigm frequencies to decipher a 75% threshold of the synchronous condition using an adaptive staircase procedure.
Proceeding from 4-Hz, this increased until the participants could no longer accurately state whether the paradigm was presenting a synchronous element (in a square arrangement) or a random arragement of four elements.
Similar results were found across 2 separate days, demonstrating stability above 21-Hz. 21 Participants (9 male)
Mean age = 27.6
2240 trials / 4 sessions (560 trials / 10 blocks) - First 2 trials of each block were considered a warmup. Elliott and Müller - Experiment 1 Looking into phenomenal awareness of the 25-Hz vs. 40-Hz by determining observers sensitivity to synchronous elements in the premask paradigm.
Observers had to indicate whether a quadrant contained synchronous elements (ranging from certain-present through to certain absent). Elliott and Müller - Experiment 2 Test spatial-attention account of synchronicity enhancement employing a RT paradigm similar to experiment 1. Elliott and Müller - Experiment 3 The target (absent/present), synchronicity (synchronous/random) and oscillation frequency varied in each trial block.
Observers reaction times (RT) and error rate was measured to the presence of the Kanizsa figure portrayed by the 90˚ corner junctions The 9 flickering crosses would reduce down to 90˚ corner junctions. These produced the Kanizsa figures. A 4-way analysis of variance (ANOVA) was performed on synchronicity, target, premask presentation frequency and presentation time.
Results showed:
Significant main effect of target, premask frequency and presentation time.
An interaction between synchronicity and target/frequency. RT was slower for the target absent condition (additional searching), as well as a slower response for the random condition.
The synchronicity effects were dependant on the oscillation frequency.
The 25 and 40-Hz premask presentation provided a significant RT advantage compared to their random counterparts yet not on the frequencies between these or greater than 40-Hz. Elliott and Müller - Conclusions (Experiment 1) The hypothesis that there would be synchronization affects throughout the gamma band is rejected.
From the results, several different hypotheses were assumed: The 25-Hz condition is thought to be down to phenomenal awareness of the pre-structure. 10 Participants (5 Males)
Mean age = 26.8
480 trials (240 for 25-Hz/240 for 40-Hz) - Blocks of 48 trials Maximum-likelihood estimate rating procedure was used. Results indicated that sensitivity to the premask was significantly higher in the 25-Hz condition, with the mean being above chance. The mean for 40-Hz was approximately at chance. In conclusion, the results from experiment 1 may have been due to strategic processing involved in allocation of spatial attention.
The 40-Hz condition supports the idea that is used the preattentive binding opertations. 20 participants (9 male, 25.4 years) - 40-Hz
17 participants (7 male, 26.2 years) - 25-Hz
40-Hz = 2560 trials / 4 sessions of 640 trials
25-Hz = 1600 trials / 4 sessions of 400 trials (less random trials) The difference was that in target-present trials, synchronous crosses could appear in a different quadrant from the Kanizsa square.
Referred to as mislocation-synchronous condition.
If spatial attention is oriented towards a non target quadrant, this will cost time relative to the RT in comparison to the random premask.
Results showed:
Synchronicity to be significant (vs. mislocation and random)
Synchronicity and Frequency interaction non significant.
t-test confirms the existance of synchronicity enhancement (Replicating experiment 1) The results demonstrate consistency between experiment 1 and 2, and that the 40-Hz condition is consistent with the synchronous-priming hypothesis.
In addition, 25-Hz proved to be advantageous for target-absent variants (experiment 2) Visual system is sensitive to external modulation, in particular when presented for preattentive target detection under 40-Hz.
RT advantage for 40-Hz frequency specific priming of neural mechanisms in early stages of visual processing.
The papers continues to ask the question of how a synchronicity advantage is actually generated - featural or figural priming.
Featural - To decipher Kanizsa square, at 40-Hz, feature coding mechanisms become temporally coactivated and primed for detection.
Figural - Whole target processing, due to temporal premask structures becoming globally synchronized. Elliott and Müller - Discussion In conclusion, perceptual organisation is enhanced by coactivation of preattentive binding mechanisms, specifically under 40-Hz. Cheadle and Bauer Rationale The BBS predicts coherent spatial structures formed by the alignment of Gabor patches (lateral connections), via neural detectors.
The experiment predicts a decrease in synchrony/asynchrony by comparing strong to weak grouping of the Gabor patches. They decided to examine the effect of spatial coherence on temporal judgements from the development of a simple generic neural model. Cheadle and Bauer They aimed to provide psychophysical and physiological evidence that humans implement a visual mechanisms to synchronize firing to bind objects. Aims The prediction was that a superior effect in temporal judgement in weak-coupling, due to the differences between different stimulus (better discrimination) Method 10 participants with normal vision completed 180 trials for each conditions.
The Gabor patches were presented in either a smooth (strong grouping) or jagged (weak) arrangement (plus an invisible, control condition).
The patches flickered at a rate of 4.49-Hz, and observers has to distinguish whether the targets - T1 and T2 - were in (synchronous) or out of phase (asynchronous), with trials being run at 25, 50, 75 and 100% phase levels.
Results showed:
Performance improved with increased phase difference (>50%).
Smooth contour's interfere with temporal judgement.
As predicted by the BBS hypothesis, there was a significant difference between strong and weak groupings, with effects peaking at 75% phase separation. Cheadle and Bauer - Experiment 1 Experiment 1 confirmed predictions about grouping strength and temporal discrimination performance Yet due to methodological problems with the yes/no structure (criterion), a follow up needed to be conducted. 4 participants took part in 240 trials for each condition (smooth and jagged).
Rather than a yes/no paradigm, a 2-interval forced-choice response criteria was used (2IFC) - assumed to be criterion free.
Due to the curved gabor patches being difficult to analyze across time, a horizontal line was used, with the targets no longer at the extremities and within a figural matrix.
This tackled potential ceiling effects of easier conditions, the frequency was increased to 6-Hz Cheadle and Bauer - Experiment 2 The observers were presented with one synchronous and one asynchronous trial, then had to identify which was which. Each lasted 1 second and was separated by 500ms. Procedure Over the 4 participants, the 50% and 75% conditions were consistently and significantly higher.
Post-hoc analysis found syn-asyn vs. asyn-syn presentation significantly influenced responses.
A bias to asyn-syn. This was found to not be due to response bias and was down to cueing by the first interval increased attention for the second interval. Cheadle and Bauer - Experiment 2 - Results Results support experiment 1, that the negative effect of spatial structure on temporal judgement is not due to the response criterion, but a true change in temporal sensitivity. The experiment has shown that the presence of grouped yet irrelevant spatial structure impedes the ability to make temporal judgements of synchrony.
They match the predictions made by the BBS hypothesis, providing further evidence.
The results showed that interference occurs most at maximal during intermediate phase differences. Cheadle and Bauer - Discussion Spatial-Attention Hypothesis - The cost affect of assuming the elements were in one quadrant, thus needing to disengage attention before searching other areas.
Synchronicity-priming hypothesis - All quadrants are processed equally (consistent with results from the 40-Hz condition) Questions arise over the 25-Hz frequency as it did not differ reliably between the target and absent conditions. They suggest a novel approach to the BBS hypothesis, namely does task irrelevant spatial structure modulate the perceived temporal structure. Elliott and Müller vs. Cheadle and Bauer The field of visual binding is very theory based, lacking objective evidence to support assumptions made by the papers. To further clarify the BBS hypothesis, brain signatures during visual discrimination tasks needs to be examined to give more concrete evidence on the topic. Yet could this support make some current research redundant if they are based on assumptions? In conclusion, perceptual organisation is enhanced by coactivation of preattentive binding mechanisms, specifically under 40-Hz.
Yet the journal has a maximum for an article of 4000 words, could this limit the detail which could let the field of visual binding suffer?
The paper is written ambiguously, this a case of bad writers of trying to convey a different story? Can either of the studies be applicable if the stimulus is artificial? It may be optimal but how useful is it if it is not real? The use of a low frequency is not justified, despite cognitive processes apparently requiring higher (such as in Elliott and Müller - a paper which Cheadle and Bauer even reference) Processing Limits Elliott and Müller VSTM and cognitive functions are limited in their capacity and time to perform essential functions. Recent research have suggested 40-Hz synchronisation in the neocortex is responsible for binding of discrete stimulus. Cheadle and Bauer Findings suggested that during visual binding at 40-Hz, that preattentive coding occurs. Expanding on this and the Binding by Synchrony Hypothesis, there was research into whether spatial structures affect temporal judgements. Processing Limits Elliott and Müller VSTM and cognitive functions are limited in their capacity and time to perform essential functions. Recent research have suggested 40-Hz synchronisation in the neocortex is responsible for binding of discrete stimulus. Cheadle and Bauer ? ? Both studies fail to use a substantial amount of participants, in particular Cheadle and Bauer
Specific aims of the studies were not obvious. Thank you for listening During visual binding, interferences can occur with irrelevant spatial stimuli.
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