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Explanatory scope : Dual-channel RECOD model

Explanatory scope : Dual-channel RECOD model. Chapter 5, Pages 186-218. Harsha KASI PhD student, Institute of Microsystems and Microelectronics EPFL. Remember. Original sustained-transient model & RECOD model share common mechanisms critical to masking

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Explanatory scope : Dual-channel RECOD model

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  1. Explanatory scope : Dual-channel RECOD model Chapter 5, Pages 186-218 Harsha KASI PhD student, Institute of Microsystems and Microelectronics EPFL

  2. Remember • Original sustained-transient model& RECOD model share common mechanisms critical to masking • Chapters 1, 2 and some additional results introduced in this chapter • Scope of 2 models by representative set of findings Explanatory scope : Dual-channel RECOD model

  3. Outline • Justification for effects and experimental findings – comparison of model simulations and psychophysical experiments • Explanatory power • Comparisons and critiques Explanatory scope : Dual-channel RECOD model

  4. Paracontrast and metacontrast suppression Flicker persisted longer in the middle band Flicker persisted longer in the double white arcs Critical flicker frequency (CFF) : former ↑ latter (target) (mask) latter ↓ former (mask) (target) Paracontrast suppression Metacontrast facilitation → Metacontrast suppression ? Sherrington (1897) Piéron (1935) – not only CFF but on brightness perception as well → metacontrast suppression • Metacontrast suppression (Brightness) – Faster transient activity by the lagging flash inhibiting the slower sustained response of the leading flash • Paracontrast suppression (CFF) – Slower sustained activity by the first stimulus reciprocally inhibiting the faster transient (flicker) by the second stimulus • 1st stimulus: higher CFF relative to the inhibited CFF of the 2nd stimulus Explanatory scope : Dual-channel RECOD model

  5. Transient masking effects 30-ms sinusoidal target grating at on-and offset of a 700-ms luminance flash mask (@ 54.8 cd/m2) transient mask overshoots Assume: 1.0 c/deg – low spatial frequency transient 7.8 c/deg – high spatial frequency sustained Peripheral transient activity by mask flash adds ‘noise’ to the ‘signal’ of transient channels and not sustained channels 1.0 c/deg: SNR or Weber ratio in transient channel ↓→ overshoots! 7.8 c/deg: only a sustained masking effect at mask onset or offset Green (1981) Explanatory scope : Dual-channel RECOD model

  6. Transient masking effects 2 Mitov et al. (1981): Overshoots inversely proportional to the spatial frequency of the grating – 2 c/deg 6 c/deg 18 c/deg Spatial frequencies at and below 6-c/deg, and with spatial frequency increase if the magnitude of transient activation decreases and that of sustained channel increases Teller et al., Matthews (1971): No overshoots with mask sizes e.g. <30’. However, with larger masks (e.g. > 60’) Low spatial frequency gratings, optimal for activating transient channels under large conditioning flash mask Breitmeyer and Julesz (1975) and Tulunay-Keesey and Bennis (1979): overshoots found in Green and Mittov’s studies depend on the rise and fall times of the mask at its on- and offsets Slowly ramped instead of abrupt on- and offsets attenuate the transient response leading to curbing the transient masking overshoots Matsumara’s (1976) work provides evidence to this ! Explanatory scope : Dual-channel RECOD model

  7. Contour and Surface dynamics – Unlumped p-pathway Metacontrast: Strongest at shorter SOA for the contour compared to the surface/brightness network (20 ms vs. 60 ms) Paracontrast: Contour network – a long-lasting suppression coupled with a strong suppression – SOA ~ -10 ms Surface network – Weaker long-lasting suppression and then enhancement Identical set of equations with different weightings associated with inhibitory and facilitatory processes Explanatory scope : Dual-channel RECOD model

  8. M/T Ratio – type B to A metacontrast Transition : mask/target energy ratio is greater than unity Difference in masking contrast thresholds Type B @ lower mask contrasts and produced by a high-gain, rapid-saturation transient-on-sustained inhibition transforms to a Type A @ high mask contrasts, produced by a low-gain linear intra-channel sustained-on-sustained inhibition superimposed on the former inter-channel inhibition Explanatory scope : Dual-channel RECOD model

  9. Left eye Right eye Dichoptic type A forward and backward masking Dichoptic type A forward masking by noise or structure is typically weaker than type A backward masking (Greenspoon and Eriksen 1968, Turvey 1973) Forward masking by structure or noise: Post-retinal transient activity can locally inhibit post-retinal sustained mask activity → less masking by integration On the contrary, backward masking: Sustained mask activity intrudes unobstructed into target’s sustained channels and @ post-retinal levels transient mask activity inhibits sustained target activity → facilitate intrusion – more masking ! Since these interactions are dichoptic, very likely exist at cortical levels Explanatory scope : Dual-channel RECOD model

  10. Monoptic – Type A forward masking stronger than type A backward masking Integration of target and mask activities occur early – photoreceptor and post-receptor neural levels prior to the centrally located sustained-transient inhibitory interactions Type A forward and backward pattern masking as well as type B para- and metacontrast are obtained dichoptically and monoptically (Alpern 1953; Michaels and Turvey 1973, etc.) Either integration in common sustained pathways or inter-channel inhibition Type B metacontrast effects ↓ in magnitude as the spatial separation between the target and mask stimuli ↑ (Alpern 1953; Breitmeyer and Horman 1981, etc.) Spatially restricted receptive fields of sustained and transient neurons & the topographical mapping between retina to the visual cortex (Brooks and Jung, 1973) Explanatory scope : Dual-channel RECOD model

  11. Reaction times – contour interactions In paracontrast, reaction times (∆RT) for target localisation increase Paracontrast: Both data and model show an inverse U function Metacontrast: constant function Paracontrast: Both data and model show an inverse U function Metacontrast: constant function Change ∆RT in reaction times due to contour interactions between the target and mask as a function of SOA for two M/T contrast ratios. The middle curve corresponds to the average of the M/T=3 and M/T=1 data. Error bars represent ±1 SE of the mean. The squares are the predictions of the model. Reproduced from Ögmen et al. 2003 Paracontrast: close examination of M/T=3 and model – an inverse W function; peaks and dips shifted w.r.t each other 2 peaks in the W-shaped function – separate contributions of inter-channel sustained-on-transient inhibition and intra-channel transient-on-transient inhibition to reduce activity of the transient channels responding target Explanatory scope : Dual-channel RECOD model

  12. Werner (1935): Metacontrast masking of a target pattern is inversely related to the orientation difference between target and mask stimuli Cortical transient as well as sustained neurons are orientation selective (Ikeda and Wright 1975; Stone and Dreher 1973) Mutual inhibition between cortical orientation-selective cells is itself orientation selective (Benvento et. al. 1972, etc.) Blurred mask does not substantially reduce metacontrast of a non-blurred target Transient channels are insensitive to high spatial frequencies and so to image blur (Growney, 1976) Single-transient paradigm (Breitmeyer and Rudd 1981): Brief mask suppresses visibility of a prolonged sustained peripheral target for several seconds Single-transient stimulus can activate transient-on-sustained inhibition, so despite the necessary 2-transient paradigm in metacontrast, contrary to Matin (1975). Activation of T-M neurons is not required, transient neuron activation by mask alone is sufficient Explanatory scope : Dual-channel RECOD model

  13. Target recoverability Addition of a second mask (M2) to a target (T) and primary metacontrast mask (M1) can lead to the recovery of visibility of the target • Two effects: • M2-T-M1 : • Target visibility recovered • No change in the visibility of prim. mask M1 • 2. T-M1-M2 : • No change in target visibility • A reduction in visibility of M1 Double disassociation, i.e., visibility and metacontrast masking effectiveness associated with sustained and transient responses Target recovery: M2 inhibits M1’s transient activity sustained-on-transient inhibition M1 reduced visibility : inter-channel transient-on-sustained inhibition by M2 Explanatory scope : Dual-channel RECOD model

  14. Comparisons and Critiques Apart from explaining various effects (Chap. 2), 2 models give adequate explanation of many variations of them as well Revised version of Weisstein et al. 1975 accounts for metacontrast – transient-on-sustained inhibition of the non-recurrent forward type For paracontrast: sustained-on-transient inhibition of the non-recurrent forward type In conformance with assumption 1 of the Breitmeyer and Ganz’s model Differs in assumptions 2 and 4, which in Breitmeyer’s models states that: Paracontrast is realised via intra-channel inhibition effected in the sustained channels, rather than Weisstein’ et. al’s corresponding assumption of inter-channel, sustained-on-transition inhibition Weisstein’s model cannot adequately account for the absence of type B metacontrast when simple reaction time or detection rather than brightness perception are used as criterion responses Explanatory scope : Dual-channel RECOD model

  15. Matin’s (1975) model with the sustained-transient model is not so similar in regard to required activation of T-M neurons T-M neurons → transient; T neurons → sustained Shorter response latency of T-M neurons compared with T neurons bears a similarity with the sustained-transient model This combined with the inter-channel inhibition is equivalent to the assumptions 1 and 3 of the sustained-transient model and the fast-inhibition hypothesis of Weisstein’s Comparisons and Critiques 2 RECOD model Converges to sustained-transient model Incorporating recent neurophysiological findings, feedback mechanisms, proposing additional feedforward, feedback-dominant phases of operation, explicit network structure and a quantitative description that can be simulated and compared directly with the experimental data Explanatory scope : Dual-channel RECOD model

  16. Comparisons and Critiques 3 Feedback structure aspect of RECOD model makes it comparable to some discussed in Chap. 4 Dual-channel aspect of RECOD model makes it significantly different from: Bridgeman’s (1971, 1977, 1978) neural-network model Ganz’s (1975) trace decay-lateral inhibition model Reeves’s (1981) non-neural models None of the neural or non-neural models incorporate the distinction between transient response components and slow sustained ones which can reciprocally inhibit each other RECOD model incorporates: Feedback (recurrent) connections as in Bridgeman’s single-channel model Dual-channel structure to avoid spatiotemporal blurring that would occur in Bridgeman’s model so that perceptual dynamics can be organised as entities and can be processed individually Explanatory scope : Dual-channel RECOD model

  17. Summary A review of psychophysical studies of spatiotemporal properties of human vision characterised by: 1. Separate pattern and movement or flicker thresholds 2. Temporal integration and persistence 3. Reaction time and effects of flicker adaptation all as a function of spatial frequency→ existence of sustained/transient channels Supported well by neurophysiological evidences – two parallel afferent pathways with similar characteristics RECOD model adequately accounts for a wide range of visual masking phenomena discussed throughout this book ! Explanatory scope : Dual-channel RECOD model

  18. Thank you for your attention Explanatory scope : Dual-channel RECOD model

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