Different Electrophysiological Correlates of Visual Awareness for Detection and Identification

Abstract

Detecting the presence of an object is a different process than identifying the object as a particular object. This difference has not been taken into account in designing experiments on the neural correlates of consciousness. We compared the electrophysiological correlates of conscious detection and identification directly by measuring ERPs while participants performed either a task only requiring the conscious detection of the stimulus or a higher-level task requiring its conscious identification. Behavioral results showed that, even if the stimulus was consciously detected, it was not necessarily identified. A posterior electrophysiological signature 200-300 msec after stimulus onset was sensitive for conscious detection but not for conscious identification, which correlated with a later widespread activity. Thus, we found behavioral and neural evidence for elementary visual experiences, which are not yet enriched with higher-level knowledge. The search for the mechanisms of consciousness should focus on the early elementary phenomenal experiences to avoid the confounding effects of higher-level processes.

Visual awareness negativity is an early neural correlate of awareness: A preregistered study with two Gabor sizes

“…Another explanation is that spatial frequency was lower in previous studies (three lines spanning a square of 1.5°) than in the Koivisto and Grassini study (10 c/deg), and N1 amplitudes increase with spatial frequency of high-contrast Gabors (Mihaylova, Hristov, Racheva, Totev, & Mitov, 2015 ). Notably, a recent study showed that VAN varies with the calibration threshold such that VAN was observed at the detection threshold but not the identification threshold (Koivisto, Grassini, Salminen-Vaparanta, & Revonsuo, 2017 ). In the study, awareness was calibrated separately to the detection and identification thresholds with the same subjective rating scale: “I did not see any stimulus,” “I saw something (but could not identify the stimulus),” “I saw the stimulus almost clearly (and could identify it),” and “I saw the stimulus clearly (and could identify it).” In a detection task, stimuli were calibrated to be rated as “I did not see any stimulus” in about 50% of the trials (not aware of the stimulus).…”

Unconscious response priming during continuous flash suppression

“…This low rate was expectable because the contrast of the stimulus was individually calibrated to a level where 50% or slightly less of the subjective responses were ‘saw nothing’. In our previous study using 4-point PAS [ 31 ], the contrast of Gabor patches was calibrated in detection task to a level in which about 50% of the stimuli received the lowest rating (‘saw nothing’). In the actual experiment, the distribution of ratings was such that ‘nothing’ was seen in 55% of trials, ‘something’ was seen in 40% of trials, ‘almost clear’ perception was present in 3% of trials, and ‘clear’ perception in 2% of trials.…”

“…We calibrated the contrast for each participant individually such that about 50% of the primes, or slightly less, would be rated as completely invisible under CFS. Under such presentation condition, we expect on basis of our earlier experience [ 31 ] that the participants would never or very rarely see the prime clearly and they would use predominantly the ‘saw nothing’ and ‘saw something’ alternatives. Therefore, we modified the subjective scale to be more consistent with participants’ experiences.…”

Temporal neural mechanisms underlying conscious access to different levels of facial stimulus contents

“…In a separate line of research (Genetti et al 2009; Tanskanen et al 2007), neural responses associated with awareness of facial identity are found to emerge relatively late around 230 ms and this effect is followed by an increase of the P300 component. The notion that conscious face detection precedes conscious face identification is further supported by a recent study on object perception (Koivisto et al 2017). In this study, stimulus hierarchy is taken into account by comparing electrophysiological correlates between two separate tasks, in which participants were required to detect the presence of a digit stimulus at threshold or to identify whether the digit was smaller or larger than 5.…”