Into the Unknown

Mental health disorders (MHD) are a rising, yet stigmatized, topic. With statistics reporting that one in five adults in the United States will be afflicted by a MHD in their lifetime, researchers have begun exploring the behavioral nuances that emerge from interactions of these individuals with persuasive technologies, mainly social media. Yet, there is a gap in the analysis pertaining to a persuasive technology that is part of their everyday lives: search engines (SE). Each day, users with MHD embark on information seeking journeys using SE. Every step of the search process for better or worse has the potential to influence a searcher’s state of mind. In this thesis work, we empirically investigate what subliminal stimulus SE present to these vulnerable individuals during their searches. We do so by utilizing an information retrieval foundation that leverages data and techniques from psychology, social media, and natural language processing. Outcomes from this work showcase open problems related to query suggestions, search engine result pages, and ranking, that the information retrieval community needs to address so that SE can better support individuals with MHD.

Subliminal perception can be predicted from prestimulus activity

Humans sometimes make accurate guesses about stimuli they report not consciously seeing—this phenomenon is known as “subliminal perception.” We asked participants (N = 31) to discriminate the location of a briefly presented low-contrast visual stimulus, and then rate how well they saw the stimulus. The behavioral accuracy of discriminating the location of a subjectively subliminal stimulus could be predicted in a trial-by-trial manner from lateralized low frequency (1–15 Hz) electroencephalographic (EEG) activity before the stimulus. This effect was observed up to 1 s before the stimulus was presented. Lateralized occipital prestimulus EEG power modulated the stimulus-evoked activity in a complex manner, but the amplitude of the stimulus-evoked electrophysiological response was not strongly modulated by subliminal objective discrimination accuracy. Signal detection analyses indicated that the participants’ capacity to discriminate subliminal stimuli lay on the same continuum as conscious vision. The results suggest that subliminal perception is not an automatic stimulus-evoked process but relies on perceptual decisions about weak perceptual signals that may be available for introspection.

Classic Stroop Color Words Produce No Stroop Effect When the Display Characteristics Are Based Upon Emotional Stroop Studies With Subliminal Presentations

Two forms of the Stroop task have produced contradictory findings regarding unconscious perceptual processing. Emotional Stroop task studies with prime words presented at an objective threshold (i.e., subliminal) produce Stroop-like effects, but comparable studies conducted with classic Stroop stimuli do not produce Stroop effects. We tested the possibility that differences in the display appearance might explain this discrepancy. Color word prime stimuli from the traditional Stroop task were used with display characteristics based upon the emotional Stroop studies. There was a Stroop effect for the relatively long prime stimulus durations (59, 87, or 108 milliseconds) but not for the brief durations (18, 24, or 38 milliseconds). Accordingly, the discrepancy in research findings cannot be attributed to simple differences in display methodology. The failure to find strong evidence of unconscious perceptual processing is consistent with the negative findings from some emotional Stroop studies that use subliminal stimulus presentations.

The Underestimated Significance of Conditioning in Placebo Hypoalgesia and Nocebo Hyperalgesia

Placebo and nocebo effects are intriguing phenomena in pain perception with important implications for clinical research and practice because they can alleviate or increase pain. According to current theoretical accounts, these effects can be shaped by verbal suggestions, social observational learning, and classical conditioning and are necessarily mediated by explicit expectation. In this review, we focus on the contribution of conditioning in the induction of placebo hypoalgesia and nocebo hyperalgesia and present accumulating evidence that conditioning independent from explicit expectation can cause these effects. Especially studies using subliminal stimulus presentation and implicit conditioning (i.e., without contingency awareness) that bypass the development of explicit expectation suggest that conditioning without explicit expectation can lead to placebo and nocebo effects in pain perception. Because only few studies have investigated clinical samples, the picture seems less clear when it comes to patient populations with chronic pain. However, conditioning appears to be a promising means to optimize treatment. In order to get a better insight into the mechanisms of placebo and nocebo effects in pain and the possible benefits of conditioning compared to explicit expectation, future studies should carefully distinguish both methods of induction.

Regulation of Local Ambient GABA Levels via Transporter-Mediated GABA Import and Export for Subliminal Learning

Perception of supraliminal stimuli might in general be reflected in bursts of action potentials (spikes), and their memory traces could be formed through spike-timing-dependent plasticity (STDP). Memory traces for subliminal stimuli might be formed in a different manner, because subliminal stimulation evokes a fraction (but not a burst) of spikes. Simulations of a cortical neural network model showed that a subliminal stimulus that was too brief (10 msec) to perceive transiently (more than about 500 msec) depolarized stimulus-relevant principal cells and hyperpolarized stimulus-irrelevant principal cells in a subthreshold manner. This led to a small increase or decrease in ongoing-spontaneous spiking activity frequency (less than 1 Hz). Synaptic modification based on STDP during this period effectively enhanced relevant synaptic weights, by which subliminal learning was improved. GABA transporters on GABAergic interneurons modulated local levels of ambient GABA. Ambient GABA molecules acted on extrasynaptic receptors, provided principal cells with tonic inhibitory currents, and contributed to achieving the subthreshold neuronal state. We suggest that ongoing-spontaneous synaptic alteration through STDP following subliminal stimulation may be a possible neuronal mechanism for leaving its memory trace in cortical circuitry. Regulation of local ambient GABA levels by transporter-mediated GABA import and export may be crucial for subliminal learning.