The biological basis of individual differences in intelligence

Evidence for an amodal domain-general object recognition ability

A general object recognition ability predicts performance across a variety of high-level visual tests, categories, and performance in haptic recognition. Does this ability extend to auditory recognition? Vision and haptics tap into similar representations of shape and texture. In contrast, features of auditory perception like pitch, timbre, or loudness do not readily translate into shape percepts related to edges, surfaces, or spatial arrangement of parts. We find that an auditory object recognition ability correlates highly with a visual object recognition ability after controlling for general intelligence, perceptual speed, low-level visual ability, and memory ability. Auditory object recognition was a stronger predictor of visual object recognition than all control measures across two experiments, even though those control variables were also tested visually. These results point towards a single high-level ability used in both vision and audition. Much work highlights how the integration of visual and auditory information is important in specific domains (e.g., speech, music), with evidence for some overlap of visual and auditory neural representations. Our results are the first to reveal a domain-general ability, o, that predicts object recognition performance in both visual and auditory tests. Because o is domain-general, it reveals mechanisms that apply across a wide range of situations, independent of experience and knowledge. As o is distinct from general intelligence, it is well positioned to potentially add predictive validity when explaining individual differences in a variety of tasks, above and beyond measures of common cognitive abilities like general intelligence and working memory.

Mitochondria as the Linchpin of General Intelligence and the Link between g, Health, and Aging

In a recent theoretical article, I proposed that the efficiency of mitochondrial functioning is the most fundamental biological mechanism contributing to individual differences in general intelligence (g; Geary, 2018). The hypothesis accommodates other contributing mechanisms at higher levels of analysis (e.g., brain networks), and is attractive because mitochondrial energy production undergirds the developmental, maintenance, and expression of these other mechanisms and provides a means to link individual differences in g to individual differences in health and successful aging in adulthood. I provide a brief summation here and a few clarifications to the original article.

Alternative to Psychological Testing

Mental tests have come in for a volley of criticisms ever since their inception at the beginning of the present century. Such criticisms came from many quarters, the promi nent among them being that the successful performance on such tests was primarily dependent upon acquired knowledge which was not equitably available to all indivi duals. The problem of bias (resulting from cultural, socio-economic and racial) has been experienced as discriminative against certain social and ethnic groups. Recent developments in psychology have mobilised efforts in the quest of alternatives to psychological testing technology. This paper examines the suitability of various attempts for measuring intelligence in terms of information processing speed (RT), inspection time (IT), STM, iconic image duration. It also reviews attempts made to diagnose functional psychosis in terms of memory processes in the form of Trace Inaccessibility (TI) of memory and assessment of personality in terms of signal detection parameters.

Electrophysiological Concomitants of Psychometric Intelligence

Since Berger developed the electroencephalograph to monitor the neural activity of the central nervous system, researchers have attempted to establish a correlation between neural activity and intellectual functioning. Many different studies have been carried out over the years. Serious methodological problems existed with many of the studies. This study followed a multivariate approach in establishing the relationship between the evoked potential of the brain and psychometric intelligence. A number of different neurophysiological measures of the auditory and visual evoked potential as well as a number of different measures of psychometric intelligence were used. Multiple correlations between 0.55 and 0.60 were obtained between the neurophysiological measures on the one hand, and the psychometric measures on the other.

Reliable activation to novel stimuli predicts higher fluid intelligence

The ability to reliably respond to stimuli could be an important biological determinant of differences in fluid intelligence (Gf). However, most electrophysiological studies of Gf employ event-related potential (ERP) measures that average brain activity over trials, and hence have limited power to quantify neural variability. Time-frequency analyses can capture cross-trial variation in the phase of neural activity, and thus can help address the importance of neural reliability to differences in Gf. This study recruited a community sample of healthy adults and measured inter-trial phase clustering (ITPC), total spectral power, and ERP amplitudes elicited by Repeated and Novel non-target stimuli during two visual oddball tasks. Condition effects, relations among the EEG measures, and relations with Gf were assessed. Early visual responses to Repeated stimuli elicited higher ITPC, yet only ITPC elicited by Novel stimuli was associated with Gf. Analyses of spectral power further highlighted the contribution of phase consistency to the findings. The link between Gf and reliable responding to changing inputs suggests an important role for flexible resource allocation in fluid intellectual skills.

The nature of the black–white difference on various psychometric tests: Spearman's hypothesis

Although the black and white populations in the United States differ, on average, by about one standard deviation (equivalent to 15 IQ points) on current IQ tests, they differ by various amounts on different tests. The present study examines the nature of the highly variable black–white difference across diverse tests and indicates the major systematic source of this between-population variation, namely, Spearman's g. Charles Spearman originally suggested in 1927 that the varying magnitude of the mean difference between black and white populations on a variety of mental tests is directly related to the size of the test's loading on g, the general factor common to all complex tests of mental ability. Eleven large-scale studies, each comprising anywhere from 6 to 13 diverse tests, show a significant and substantial correlation between tests' g loadings and the mean black–white difference (expressed in standard score units) on the various tests. Hence, in accord with Spearman's hypothesis, the average black–white difference on diverse mental tests may be interpreted as chiefly a difference in g, rather than as a difference in the more specific sources of test score variance associated with any particular informational content, scholastic knowledge, specific acquired skill, or type of test. The results of recent chronometric studies of relatively simple cognitive tasks suggest that the g factor is related, at least in part, to the speed and efficiency of certain basic information-processing capacities. The consistent relationship of these processing variables to g and to Spearman's hypothesis suggests the hypothesis that the differences between black and white populations in the rate of information processing may account for a part of the average black–white difference on standard IQ tests and their educational and occupational correlates.

Response-time corrected averaging of event-related potentials

OBJECTIVE

The present study presents a novel approach to averaging of event-related potentials (ERPs). Acknowledging latency variability of late ERP components as related to performance fluctuations across trials should improve the assessment of late portions of the ERP.

METHODS

Prior to the averaging procedure stimulus-to-response epochs in the electroencephalogram (EEG) were expanded/compressed in time to match mean RT in a certain condition and participant. By means of several mathematical functions RT variability was differentially distributed over late vs. early portions of the ERP. Data from 20 participants from two conditions of an identity-based priming task were analyzed using traditional stimulus- and response-locked averaging, as well as four different RT-corrected averaging procedures.

RESULTS

Area under the curve as an index of precision of LPC assessment was reliably enhanced for certain RT-corrected procedures relative to traditional ERP averaging. Moreover, a priming effect on amplitude of a distinct LPC subcomponent which could not be confirmed with traditional stimulus-locked averaging was reliably born out using a cubic RT-correction procedure.

CONCLUSIONS

RT-corrected ERP averaging can outperform traditional ERP averaging in the assessment of late portions of the ERP, and experimental effects upon.

SIGNIFICANCE

Cognitive ERP researchers may take advantage of the improved capability of RT-corrected averaging to establish experimental effects on amplitudes in the late ERP range.

Timing Performance as a Predictor of Psychometric Intelligence as Measured by Speed and Power Tests

In the present study, the relationship between timing performance and psychometric intelligence as measured by a speed and a power test of intelligence was examined. For this purpose performance on the Zahlen-Verbindungs-Test (ZVT), the Wiener Matrizen-Test (WMT), seven psychophysical temporal tasks, and the Hick reaction-time paradigm was obtained in 190 participants. Correlational and principal component analyses suggested a unitary timing mechanism referred to as temporal g. Performance on single temporal tasks and individual factor scores on temporal g were substantially related to both speed and power measures of psychometric intelligence. Temporal g exhibited higher sensitivity to the prediction of performance on the power test than on the speed test. Furthermore, stepwise multiple regression analysis and commonality analysis revealed that timing performance provides a more powerful predictor of psychometric intelligence than traditional reaction-time measures derived from the Hick paradigm. These findings support the notion that the temporal resolution capacity of the brain as assessed with psychophysical temporal tasks reflects an essential property of brain functioning, which is relevant to a wide range of intelligence-related aspects of neural information processing.

Genetics of human prefrontal function

Evolution of the prefrontal cortex was an essential precursor to civilization. During the past decade, it became increasingly obvious that human prefrontal function is under substantial genetic control. In particular, heritability studies of frontal lobe-related neuropsychological function, electrophysiology and neuroimaging have greatly improved our insight. Moreover, the first genes that are relevant for prefrontal function such as catechol-O-methyltransferase (COMT) are currently discovered. In this review, we summarize the present knowledge on the genetics of human prefrontal function. For historical reasons, we discuss the genetics of prefrontal function within the broader concept of general cognitive ability (intelligence). Special emphasis is also given to methodological concerns that need to be addressed when conducting research on the genetics of prefrontal function in humans.

Differences in EEG current density related to intelligence

Differences in current density between high intelligent (IQ=127), and low intelligent individuals (IQ=87), while solving two oddball tasks (auditive and visual) were analyzed with low resolution brain electromagnetic tomography (LORETA). In highly intelligent individuals a decrease in the volume of activated cortical gray matter between the P300 onset and the P300 peak amplitude was observed. The EEG of low intelligent individuals showed a reverse pattern of cortical activity. In the auditive oddball task the decrease in the activated cortical volume in high intelligent individuals was accompanied by an increase in current density, and a more left hemispheric source location at maximum current density. The results suggest that high intelligent individuals more efficiently distributed their cognitive resources needed to cope with the oddball tasks.

Correlations between ERP parameters and intelligence: a reconsideration

The present study investigated differences in ERP parameters related to intelligence. For that purpose 74 individuals (Intelligence: M=107; S.D.=12; range 73-135), of average creativity passively listened to two tones and performed two auditory, and two visual oddball tasks while their EEG was recorded. The approximate entropy parameters, peak latencies and amplitudes were determined. The correlation coefficients indicated that in the attended conditions, the more intelligent individuals showed more regular ERP waveforms than less intelligent individuals. It was further found that less intelligent individuals showed increased P300 latencies and reduced amplitudes. The differences were explained with a more specific engagement of neural networks in more intelligent individuals.

Spatiotemporal organization of brain dynamics and intelligence: an EEG study in adolescents

This study investigated relationships between global dynamics of brain electric activity and intelligence. EEG was recorded monopolarly from 10 symmetric leads (10-20 system) in 37 (17 males) healthy subjects (mean age 13.7 years) at rest and during performance of two visually presented cognitive tasks, verbal (semantic grouping) and spatial (mental rotation). On another occasion, the subjects were administered the Intelligence Structure Test (IST). Both total IST score and some individual subtests of specific abilities showed significant positive correlations with EEG coherence in the theta band and significant negative relationships with EEG dimension, a measure of complexity and unpredictability of neural oscillatory dynamics underlying the EEG time series. Furthermore, EEG coherence and dimensional complexity were inversely related. Taken together, these EEG metrics accounted for over 30% of the variability of the total IST score in this sample. No significant effects of the task type (spatial vs. verbal) or specific abilities were observed. Long-distance theta coherence between frontal and parieto-occipital areas showed the most consistent relationship with cognitive abilities. The results suggest that order to chaos ratio in task-related brain dynamics may be one of the biological factors underlying individual differences in cognitive abilities in adolescents.

Individual differences in the use of depth cues: implications for computer- and video-based tasks

This paper reports an experimental study of individual differences in the performance of computer-stimulated and 'real world' versions of an interactive depth perception task. The availability of depth cues (disparity, accommodation, luminance, and texture) was manipulated. Results indicate that ability to perceive depth using binocular cues is not associated with ability to perceive depth using monocular cues. Further, ability to integrate cues was a strong correlate of depth perception when multiple cues were available, and appeared more important in this respect than ability to use individual depth cues. Correlations between psychometric measures of fluid intelligence/spatial ability and depth perception were generally weak, but consistent with the broader pattern of results, suggesting that individual differences in depth perception should currently be regarded as 'cue specific'. Hypothesized facilitative effects of paradoxical monocular stereopsis were not found.