The train of thought: How our brain responds to the environment whilst we are thinking in terms of mental images or an inner voice

Mario Villena-González
Laboratorio de Neurodinámica Básica y Aplicada, Pontificia Universidad Católica de Chile, Chile

(cc) Violeta Kallens y Mario Villena.

(cc) Violeta Kallens y Mario Villena.

When our attention is internally oriented toward self-generated thoughts, brain processing to external stimuli becomes attenuated. Studies have shown that visual imagery and auditory/verbal thoughts activate brain regions involved in visual or auditory perception, respectively. However, the potential effects that different mental contents might have on attentional processing were unknown. A recent study from our laboratory showed that visual imagery decreases visual processing to external stimuli to a greater extent than auditory/verbal thoughts. These results demonstrate that different kinds of thoughts differentially affect our attentional and neural processing of external stimuli.

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Frequently, our attention shifts from the environment toward self-generated thoughts which take possession of our conscious experience. This can spontaneously occur while we perform everyday tasks such as reading a book, during which we sometimes notice that our mind wanders and thinks about something else. In addition, our attention can also be voluntarily guided towards our thoughts as a result of personal goals, for example, when remembering where some lost object was left or planning our next vacation.

Either way, attending to our inner thoughts has a significant cost: it diminishes our attention to the environment. This means that during a mind wandering episode, our brain decreases its ability to process information from the external world. This has been demonstrated using different techniques to investigate neural sensory processing, such as “event-related potentials” (ERP) and electrical oscillations in the frequency band between 8 and 12 Hz, known as “alpha band”. Specifically, ERPs are voltage deflections generated by the sum of postsynaptic potentials in the cerebral cortex. Thus, the amplitude of the first wave (called P1) of visual ERPs has a direct relationship with cortical visual processing. Conversely, the amplitude of the alpha band (spectral power) in the visual cortex is correlated with cortical inhibition. Using these techniques, studies have shown that when attention is internally oriented, there is a decrease in brain capacity to process external stimuli (Schooler et al., 2011).

However, methodological difficulties arising from the inter-subject variability in the ability to introspectively access mental states or from the difficulty in determining the time onset of a mind wandering episode have limited the study of this phenomenon. Therefore, to date no studies have addressed the question of whether such sensory processing reduction is differentially affected depending on the kind of attended mental content. This is important because it would imply that not all kinds of internal attentional orientation are equally disruptive with attention to the external world. This would certainly have consequences in activities where this attentional phenomenon has negative effects, such as paying attention in the classroom or driving a car.

A recent study from our laboratory addressed this question by developing a new methodological approach to explore the sensory response of the brain during attentional states oriented to mental images or inner voice (Villena-Gonzalez, Lopez & Rodríguez, 2016). Previous studies have shown that visual imagery activates brain regions related to visual perception (Kosslyn, Ganis & Thompson, 2001) and inner speech activates regions related to auditory perception, but not visual regions (McGuire et al., 1996). This suggests that thoughts use different cortical processing resources depending on the format of the mental representation, which could affect attention to external stimuli if they use the same sensory modality. Based on this evidence, we hypothesized that people immersed in visual imagery could have a greater attenuation in sensory processing of visual stimuli than when they are generating inner speech, due to competition for cortical processing resources.

This study used cues that instructed the participants to either pay attention to stimuli that appeared on the screen (“external task”), imagine visual thoughts, or generate inner speech, as appropriate (Figure 1). For the three conditions the amplitude of the P1 component of the visual ERP and the spectral power of alpha in the visual areas were calculated.

Figura 1

Figure 1.- Experimental design. At the beginning of each trial, when the word “imagine” appears followed by the green circle, participants were instructed to think about any mental image, avoiding auditory elements. If the word “speech” was followed by a green circle, participants were instructed to think using only their inner voice. Finally, if any of the two words were followed by a red circle, participants were instructed to focus on the appearing stimuli (checkerboard), count the squares and mark the tenth one using a button.

The main results are summarized in Figure 2. Both tasks involving inward attention showed a reduction in visual processing compared to the external task. This can be observed as a reduction in the amplitude of P1 and the greater power in the alpha band. Furthermore, we also found that visual imagery induces an even greater attenuation than auditory-verbal thinking.

Figure 2

Figure 2.- Main results. A) Analysis of ERPs and alpha power were calculated over electrodes corresponding to the visual areas of the brain (highlighted in red on brain image). B) For the three conditions, the ERP evoked by the checkerboard stimulus is shown (left) as well as the amplitude of the P1 component (right). Conditions involving inward attention reduce the amplitude of P1 to a larger extent than external attention. In addition, the visual imagery condition decreases the amplitude even more than the inner speech condition. C) For the three conditions, spectral power is shown from 1 to 20Hz (left) as well as the relative power in the alpha band (right). Conditions involving inward attention increase the amplitude of alpha compared to external attention. Importantly, visual imagery shows even greater alpha power than inner speech. (Graphs show means and standard error, * p <0.05).

Previous studies have shown that attention to one sensory modality increases the activity in the sensory cortices associated with the attended modality while simultaneously decreasing the activity of sensory cortices related to the ignored modality, thereby supporting the hypothesis that attention could be mediated by either supramodal attentional control processes or separable but connected systems (Spence, 2014). In the present study, the results showed, on the one hand, that there is a degree of overall sensory attenuation during inward attentional orientation, which suggests that supramodal control processes might be operating. However, we also observed modulations depending on the content of thought (in this case, visual), selectively interfering with brain areas related to the same sensory modality. In summary, both supramodal and modality-specific processes might be operating when we pay attention to internal thoughts (Figure 3).

Figure 3

Figure 3.- Schematic illustration of the likely attentional control processes involved in internally oriented attention. Sensory processing to a visual stimulus is higher when attention is focused on external demands. When attention is directed towards internal thoughts, there is a reduction in processing regardless of the content of thought. This could be mediated by supramodal attentional control processes or general domain processes. Finally, if the thought is visually represented as mental images, this reduction in brain processing is even greater than when thoughts are represented as auditory inner speech. This last result suggests that processes specific to the visual modality are also affecting attentional control.

The results of this study reveal, for the first time, the differential effects that the content of thought has on the neural response to sensory stimuli. In addition, this new methodological approach of using “instructed-thinking tasks” gives rise to new ways of exploring the potential effects that mental contents may have on attention and other cognitive processes.


Kosslyn, S. M., Ganis, G., & Thompson, W. L. (2001). Neural foundations of imagery. Nature Reviews Neuroscience, 2, 635-642.

McGuire, P. K., Silbersweig, D. A., Murray, R. M., David, A. S., Frackowiak, R. S., & Frith, C. D. (1996). Functional anatomy of inner speech and auditory verbal imagery. Psychological Medicine, 26, 29-38.

Schooler, J. W., Smallwood, J., Christoff, K., Handy, T. C., Reichle, E. D., & Sayette, M. A. (2011). Meta-awareness, perceptual decoupling and the wandering mind. Trends in Cognitive Science, 15, 319-326.

Spence, C. (2014). Orienting attention: A crossmodal perspective. In: A. C. Nobre y S. Kastner (Eds.), The Oxford Handbook of Attention. Oxford University Press.

Villena-González, M., López, V., & Rodríguez, E. (2016). Orienting attention to visual or verbal/auditory imagery differentially impairs the processing of visual stimuli. Neuroimage, 132, 71-78.

Manuscript received on February 22nd, 2016.
Accepted on March 10th, 2016.

This is the English version of
Villena-González, M. (2016). El tren de los pensamientos: cómo responde nuestro cerebro al entorno mientras evocamos imágenes mentales o generamos un discurso interno. Ciencia Cognitiva, 10:1, 19-22.

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