Biology subjects

Kandemir, G.

Publications and source records attributed to Kandemir, G..

8 recordsLinked to original sources

bioRxiv2026

Repetition Dissociates Pointer and Content-based Representations in Visual Working Memory: Contrasting the CDA with Multivariate Shape Decoding

Memorizing a new phone number or address is hard at first, but becomes easier with repetition, as information shifts from working memory to long-term memory. Here we investigated how repetition affects the storage and transition of different aspects of mnemonic information by comparing univariate neural markers of active object storage with multivariate decoding of memory content. Thirty participants encoded lateralized stimuli from a continuous shape space into memory. Memory items were repeated six times in a row to induce learning. In line with earlier work, EEG recordings revealed that repetition led to a reduction in contralateral delay activity (CDA), a measure of active storage that has been taken to reflect a pointer-like representation of the individual object or its original source. In contrast, shape decoding during the retention and also after an impulse perturbation remained constant across repetitions. These results suggest that learning over repetitions reflects the abolishment of active and individuated object memory representations while passive, source-independent memory representations are retained. HighlightsO_LIWe recorded EEG as participants maintained a shape item, drawn form a continuous shape space, in working memory. These memory items repeated six times in a row before they changed to a new item. C_LIO_LIWe replicated the well-known CDA decrement effect from repetition, a process thought to index participants shift from relying on working memory to long term memory. C_LIO_LIMultivariate decoding of shape identity was stable across repetitions, neither decreasing nor reliably increasing with repetition. This was true both after encoding and also following a task irrelevant ping. C_LIO_LIThese results dissociate the popular CDA ERP, a measure of memory load, from actual working memory content decodable via multivariate approaches. C_LI

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bioRxiv2025

The neural correlates of parallel and serial search

For almost half a century, target-distractor similarity has been known to induce different visual search modes. When a target is highly salient, it can pop out, suggesting parallel processing of all items irrespective of set size. By contrast, high similarity among items requires item-by-item comparison with an attentional template, a characteristic of serial search. Despite this long-standing distinction, little is known about the neural correlates of search modes, as typical differences in visual displays confound interpretation. Here, we contrasted the neural correlates of serial and parallel search under visually identical displays. Across distinct blocks, we biased 24 participants (21 female) toward parallel or serial search by varying target-distractor similarity, thereby directing attentional focus toward a single feature or conjunction. Embedded among inducer trials, test trials were visually identical across all blocks and afforded both parallel and serial search. Behavioral analyses confirmed successful induction of distinct search modes during test trials. EEG decoding reliably discriminated search modes and these neural patterns generalized across inducer and test trials. Attentional deployment toward the target differed across search modes, revealing topographical differences in target location representations. The strength of target-location representations correlated with response times, indicating that during parallel test trials participants switched search strategies when the target was not detected early. Moreover, target representations diverged between search modes: a temporally stable pattern emerged during serial search, suggesting reliance on working memory, whereas parallel search was characterized by more dynamic representations, likely reflecting prioritization of the relevant feature. These findings demonstrate that search history shapes search mode, giving rise to clearly distinct neural dynamics even under visually identical stimulation.

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bioRxiv2025

When do peripherally encoded memories generalize in visual working memory?

Visual working memory has been shown to transition from location-specific to spatially generalized representations. It remains unclear what the time course of this generalization is and to which information processing stages and signals it is linked - specifically whether it is tied to sensory or report stages. Here we used EEG to investigate spatial generalisation of peripherally encoded stimuli before and after an item was selected for report. Thirty healthy participants memorized two orientations presented serially at fifteen degrees eccentricity to the left and right of central fixation. One of the two items was subsequently cued as task relevant. During the delays, impulse signals were inserted to trace memoranda in potentially activity-reduced states. We observed that prior to the cue, patterns of activity transitioned from location-specific spatially generalized states from approximately 400-600 ms after onset in broadband voltage but not in oscillatory alpha signals. The picture was more mixed after the selection cue, with some evidence for locations-specific and spatially generalized codes in both alpha and broadband voltage. We conclude that the first stages of spatial generalization occur at sensory, pre-report levels of processing, while late stages are associated with selecting response-relevant spatial features.

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bioRxiv2023

Comparing Neural Correlates of Memory Encoding and Maintenance for Foveal and Peripheral Stimuli

Visual working memory is believed to rely on top-down attentional mechanisms that sustain active sensory representations in early visual cortex. However, both bottom-up sensory input and top-down attentional modulations thereof have been shown to be biased towards the fovea, at the expense of the periphery, and initially peripheral percepts may even be assimilated by foveal processing. This raises the question whether and how visual working memory differs for central and peripheral input. To address this, we conducted a delayed orientation recall task in which an orientation stimulus was presented either at the center of the screen or at 15{degrees} eccentricity to the left or right. Response accuracy, electroencephalographic (EEG) activity and gaze position were recorded from 30 participants. Accuracy was slightly but significantly higher for foveal versus peripheral memories. Orientation decoding analyses of the EEG signals revealed a clear dissociation between early sensory and later maintenance signals. While sensory signals were clearly decodable for the central location, they were not for peripheral locations. In contrast, maintenance signals were decodable to an equal extent for both foveal and peripheral memories, suggesting comparable top-down components regardless of eccentricity. Moreover, while memory representations were initially spatially specific and reflected in voltage fluctuations, later in the maintenance period they generalized across locations, as emerged in alpha oscillations, thus revealing a dynamic transformation within memory. Moreover, these transformed representations remained accessible through sensory impulse-driven perturbations, unveiling the underlying memory state. The eccentricity-driven dissociation between disparate sensory and common maintenance representations indicates that storage activity patterns as measured by EEG reflect signals beyond primary visual cortex.

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bioRxiv2023

Maintenance of colour memoranda in activity-quiescent working memory states: Evidence from impulse perturbation

The neural mechanisms underlying working memory maintenance pose a challenge for investigation, as sustained neural activity may not always be observable. To address this, the method of impulse perturbation has been employed to examine memorized information during activity-quiescent periods. However, this approach has mainly focused on spatially localized or referenced stimuli, leaving it unclear whether non-spatial memoranda share similar neural maintenance mechanisms. This study aimed to fill this gap by applying the impulse perturbation method to working memory for colours, which are inherently non-spatial stimuli. EEG data from 30 participants performing a delayed match-to-sample task were analysed, with one of the presented items being retro-cued as task-relevant. Our findings indicate that both cued and uncued colours could be decoded from impulse-evoked activity, in contrast to previous reports on working memory for orientation gratings. Additionally, we explored colour decoding from ongoing oscillations in the alpha band and discovered that cued items could be decoded, potentially influenced by attention, whereas uncued items could not. These results suggest subtle differences between the representation of colours and stimuli with spatial properties. However, they also demonstrate that both types of information can be accessed through visual impulse perturbation, regardless of their specific neural states.

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bioRxiv2023

Impulse perturbation reveals cross-modal access to sensory working memory through learned associations

We investigated if learned associations between visual and auditory stimuli can afford full cross-modal access to working memory. Previous research using the impulse perturbation technique has shown that cross-modal access to working memory is one-sided; visual impulses reveal both auditory and visual memoranda, but auditory impulses do not seem to reveal visual memoranda (Wolff et al., 2020b). Our participants first learned to associate six auditory pure tones with six visual orientation gratings. Next, a delayed match-to-sample task for the orientations was completed, while EEG was recorded. Orientation memories were recalled either via their learned auditory counterpart, or were visually presented. We then decoded the orientation memories from the EEG responses to both auditory and visual impulses presented during the memory delay. Working memory content could always be decoded from visual impulses. Importantly, through recall of the learned associations, the auditory impulse also evoked a decodable response from the visual WM network, providing evidence for full cross-modal access. We also observed that after a brief initial dynamic period, the representational codes of the memory items generalized across time, as well as between perceptual maintenance and long-term recall conditions. Our results thus demonstrate that accessing learned associations in long-term memory provides a cross-modal pathway to working memory that seems to be based on a common coding scheme.

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bioRxiv2022

Concurrent maintenance of both veridical and transformed working memory representations

In the dynamic environment we live in, the already limited information that human working memory can maintain needs to be constantly updated to optimally guide behaviour. Indeed, previous studies showed that leading up to a response, representations maintained in working memory representations are transformed continuously. This goes hand-in-hand with the removal of task-irrelevant items. However, does such removal also include the representations of stimuli as they were originally, prior to transformation? Here we assessed the neural representation of task-relevant transformed representations, and the no-longer-relevant veridical representations they originated from. We applied multivariate pattern analysis to electroencephalographic data during maintenance of orientation gratings with and without mental rotation. During maintenance, we perturbed the representational network by means of a visual impulse stimulus, and were thus able to successfully decode veridical as well as imaginary, transformed orientation gratings from impulse-driven activity. The impulse response reflected only task-relevant (cued), but not task-irrelevant (uncued) items, suggesting that the latter were quickly discarded from working memory. By contrast, even though the original cued orientation gratings were also no longer task-relevant after mental rotation, these items continued to be represented next to the rotated ones, in different representational formats. This seemingly inefficient use of scarce working memory capacity was associated with reduced probe response times and may thus serve to increase precision and flexibility in guiding behaviour in dynamic environments.

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bioRxiv2019

Impulse responses reveal unimodal and bimodal access to visual and auditory working memory

It is unclear to what extent sensory processing areas are involved in the maintenance of sensory information in working memory (WM). Previous studies have thus far relied on finding neural activity in the corresponding sensory cortices, neglecting potential activity-silent mechanisms such as connectivity-dependent encoding. It has recently been found that visual stimulation during visual WM maintenance reveals WM-dependent changes through a bottom-up neural response. Here, we test whether this impulse response is uniquely visual and sensory-specific. Human participants (both sexes) completed visual and auditory WM tasks while electroencephalography was recorded. During the maintenance period, the WM network was perturbed serially with fixed and task-neutral auditory and visual stimuli. We show that a neutral auditory impulse-stimulus presented during the maintenance of a pure tone resulted in a WM-dependent neural response, providing evidence for the auditory counterpart to the visual WM findings reported previously. Interestingly, visual stimulation also resulted in an auditory WM-dependent impulse response, implicating the visual cortex in the maintenance of auditory information, either directly, or indirectly as a pathway to the neural auditory WM representations elsewhere. In contrast, during visual WM maintenance only the impulse response to visual stimulation was content-specific, suggesting that visual information is maintained in a sensory-specific neural network, separated from auditory processing areas.\n\nSignificance StatementWorking memory is a crucial component of intelligent, adaptive behaviour. Our understanding of the neural mechanisms that support it has recently shifted: rather than being dependent on an unbroken chain of neural activity, working memory may rely on transient changes in neuronal connectivity, which can be maintained efficiently in activity-silent brain states. Previous work using a visual impulse stimulus to perturb the memory network has implicated such silent states in the retention of line orientations in visual working memory. Here, we show that auditory working memory similarly retains auditory information. We also observed a sensory-specific impulse response in visual working memory, while auditory memory responded bi-modally to both visual and auditory impulses, possibly reflecting visual dominance of working memory.

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